Implement title_format

This implements the title_format command, with a new placeholder %shell
which gets substituted with the view type (xwayland, xdg_shell_v6 or
wl_shell).

Example config:

    for_window [title=".*"] title_format %title (class=%class instance=%instance shell=%shell)

sway/meson.build

@@ -60,6 +60,7 @@ sway_sources = files(
 	'commands/set.c',
 	'commands/split.c',
 	'commands/swaybg_command.c',
+	'commands/title_format.c',
 	'commands/workspace.c',
 	'commands/ws_auto_back_and_forth.c',