Add support for simple menus usable with mouse or keyboard. New command

display-menu shows a menu (bound to the mouse on status line by default)
and a couple of extra formats for the default menus.

Makefile

@@ -15,6 +15,7 @@ SRCS=	alerts.c \
 	cmd-confirm-before.c \
 	cmd-copy-mode.c \
 	cmd-detach-client.c \
+	cmd-display-menu.c \
 	cmd-display-message.c \
 	cmd-display-panes.c \
 	cmd-find-window.c \
@@ -86,6 +87,7 @@ SRCS=	alerts.c \
 	layout-set.c \
 	layout.c \
 	log.c \
+	menu.c \
 	mode-tree.c \
 	names.c \
 	notify.c \