Allow replacing each of the many sets of separate foo-{fg,bg,attr}

options with a single foo-style option. For example:

    set -g status-fg yellow
    set -g status-bg red
    set -g status-attr blink

Becomes:

    set -g status-style fg=yellow,bg=red,blink

The -a flag to set can be used to add to rather than replace a style. So:

    set -g status-bg red

Becomes:

    set -ag status-style bg=red

Currently this is fully backwards compatible (all *-{fg,bg,attr} options
remain) but the plan is to deprecate them over time.

From Tiago Cunha.

grid.c

@@ -37,7 +37,6 @@
 
 /* Default grid cell data. */
 const struct grid_cell grid_default_cell = { 0, 0, 8, 8, (1 << 4) | 1, " " };
-const struct grid_cell grid_marker_cell = { 0, 0, 8, 8, (1 << 4) | 1, "_" };
 
 #define grid_put_cell(gd, px, py, gc) do {			\
 	memcpy(&gd->linedata[py].celldata[px], 			\