Rewrite of choose mode, both to simplify and tidy the code and to add

some modern features.

Now the common code is in mode-tree.c, which provides an API used by the
three modes now separated into window-{buffer,client,tree}.c. Buffer
mode shows buffers, client mode clients and tree mode a tree of
sessions, windows and panes.

Each mode has a common set of key bindings plus a few that are specific
to the mode. Other changes are:

- each mode has a preview pane: for buffers this is the buffer content
  (very useful), for others it is a preview of the pane;

- items may be sorted in different ways ('O' key);

- multiple items may be tagged and an operation applied to all of them
  (for example, to delete multiple buffers at once);

- in tree mode a command may be run on the selected item (session,
  window, pane) or on tagged items (key ':');

- displayed items may be filtered in tree mode by using a format (this
  is used to implement find-window) (key 'f');

- the custom format (-F) for the display is no longer available;

- shortcut keys change from 0-9, a-z, A-Z which was always a bit weird
  with keys used for other uses to 0-9, M-a to M-z.

Now that the code is simpler, other improvements will come later.

Primary key bindings for each mode are documented under the commands in
the man page (choose-buffer, choose-client, choose-tree).

Parts written by Thomas Adam.

screen-write.c

@@ -362,6 +362,9 @@ screen_write_copy(struct screen_write_ctx *ctx, struct screen *src, u_int px,
 	struct grid_cell	 gc;
 	u_int		 	 xx, yy, cx, cy, b;
 
+	if (nx == 0 || ny == 0)
+		return;
+
 	cx = s->cx;
 	cy = s->cy;
 
@@ -384,6 +387,121 @@ screen_write_copy(struct screen_write_ctx *ctx, struct screen *src, u_int px,
 	}
 }
 
+/* Draw a line on screen. */
+void
+screen_write_line(struct screen_write_ctx *ctx, u_int nx, int left, int right)
+{
+	struct screen		*s = ctx->s;
+	struct grid_cell	 gc;
+	u_int			 cx, cy, i;
+
+	cx = s->cx;
+	cy = s->cy;
+
+	memcpy(&gc, &grid_default_cell, sizeof gc);
+	gc.attr |= GRID_ATTR_CHARSET;
+
+	screen_write_putc(ctx, &gc, left ? 't' : 'q');
+	for (i = 1; i < nx - 1; i++)
+		screen_write_putc(ctx, &gc, 'q');
+	screen_write_putc(ctx, &gc, right ? 'u' : 'q');
+
+	screen_write_cursormove(ctx, cx, cy);
+}
+
+/* Draw a box on screen. */
+void
+screen_write_box(struct screen_write_ctx *ctx, u_int nx, u_int ny)
+{
+	struct screen		*s = ctx->s;
+	struct grid_cell	 gc;
+	u_int			 cx, cy, i;
+
+	cx = s->cx;
+	cy = s->cy;
+
+	memcpy(&gc, &grid_default_cell, sizeof gc);
+	gc.attr |= GRID_ATTR_CHARSET;
+
+	screen_write_putc(ctx, &gc, 'l');
+	for (i = 1; i < nx - 1; i++)
+		screen_write_putc(ctx, &gc, 'q');
+	screen_write_putc(ctx, &gc, 'k');
+
+	screen_write_cursormove(ctx, cx, cy + ny - 1);
+	screen_write_putc(ctx, &gc, 'm');
+	for (i = 1; i < nx - 1; i++)
+		screen_write_putc(ctx, &gc, 'q');
+	screen_write_putc(ctx, &gc, 'j');
+
+	for (i = 1; i < ny - 1; i++) {
+		screen_write_cursormove(ctx, cx, cy + i);
+		screen_write_putc(ctx, &gc, 'x');
+	}
+	for (i = 1; i < ny - 1; i++) {
+		screen_write_cursormove(ctx, cx + nx - 1, cy + i);
+		screen_write_putc(ctx, &gc, 'x');
+	}
+
+	screen_write_cursormove(ctx, cx, cy);
+}
+
+/* Write a preview version of a window. */
+void
+screen_write_preview(struct screen_write_ctx *ctx, struct screen *src, u_int nx,
+    u_int ny)
+{
+	struct screen		*s = ctx->s;
+	struct grid_cell	 gc;
+	u_int			 cx, cy, px, py;
+
+	cx = s->cx;
+	cy = s->cy;
+
+	/*
+	 * If the cursor is on, pick the area around the cursor, otherwise use
+	 * the top left.
+	 */
+	if (src->mode & MODE_CURSOR) {
+		px = src->cx;
+		if (px < nx / 3)
+			px = 0;
+		else
+			px = px - nx / 3;
+		if (px + nx > screen_size_x(src)) {
+			if (nx > screen_size_x(src))
+				px = 0;
+			else
+				px = screen_size_x(src) - nx;
+		}
+		py = src->cy;
+		if (py < ny / 3)
+			py = 0;
+		else
+			py = py - ny / 3;
+		if (py + ny > screen_size_y(src)) {
+			if (ny > screen_size_y(src))
+				py = 0;
+			else
+				py = screen_size_y(src) - ny;
+		}
+	} else {
+		px = 0;
+		py = 0;
+	}
+
+	screen_write_copy(ctx, src, px, src->grid->hsize + py, nx, ny, NULL,
+	    NULL);
+
+	if (src->mode & MODE_CURSOR) {
+		grid_view_get_cell(src->grid, src->cx, src->cy, &gc);
+		gc.attr |= GRID_ATTR_REVERSE;
+		screen_write_cursormove(ctx, cx + (src->cx - px),
+		    cy + (src->cy - py));
+		screen_write_cell(ctx, &gc);
+	}
+}
+
 /* Set up context for TTY command. */
 static void
 screen_write_initctx(struct screen_write_ctx *ctx, struct tty_ctx *ttyctx)