add seat sub command 'xcursor_theme'

New 'seat <name> xcursor_theme <theme> [<size>]' command that
configures the default xcursor theme.

The default seat's xcursor theme is also propagated to XWayland, and
exported through the XCURSOR_THEME and XCURSOR_SIZE environment
variables. This is done every time the default seat's configuration is
changed.

sway/meson.build

@@ -89,6 +89,7 @@ sway_sources = files(
 	'commands/seat/fallback.c',
 	'commands/seat/hide_cursor.c',
 	'commands/seat/pointer_constraint.c',
+	'commands/seat/xcursor_theme.c',
 	'commands/set.c',
 	'commands/show_marks.c',
 	'commands/smart_borders.c',