Implement focus_follows_mouse

Also contains two other small changes:

- Clicking any button will focus the container clicked (not just left)
- Remove seamless_mouse (doesn't make sense on wlroots)

sway/meson.build

@@ -13,6 +13,7 @@ sway_sources = files(
 	'commands/exec.c',
 	'commands/exec_always.c',
 	'commands/focus.c',
+	'commands/focus_follows_mouse.c',
 	'commands/kill.c',
 	'commands/include.c',
 	'commands/input.c',