bindsym/code: add group support

This adds support for specifying a binding for a specific group. Any
binding without a group listed will be available in all groups. The
priority for matching bindings is as follows: input device, group, and
locked state.

For full compatibility with i3, this also adds Mode_switch as an alias
for Group2. Since i3 only supports this for backwards compatibility
with older versions of i3, it is implemented here, but not documented.

include/sway/config.h

@@ -53,6 +53,7 @@ struct sway_binding {
 	list_t *keys; // sorted in ascending order
 	list_t *syms; // sorted in ascending order; NULL if BINDING_CODE is not set
 	uint32_t modifiers;
+	xkb_layout_index_t group;
 	char *command;
 };