tree-wide: use enum types/constants where appropriate

- add LAB_WINDOW_TYPE_INVALID in place of literal -1
- document more clearly that enum lab_view_criteria is a bitset
- other one-off replacements of integer values/types for consistency

Note: variables of type enum lab_view_criteria are already used
extensively throughout the code to contain combinations of the declared
enum values. I am not introducing any new usage here, just changing the
single uint32_t to be consistent with all the other usages.

include/config/rcxml.h

@@ -173,7 +173,7 @@ struct rcxml {
 		bool show;
 		bool preview;
 		bool outlines;
-		uint32_t criteria;
+		enum lab_view_criteria criteria;
 		struct wl_list fields;  /* struct window_switcher_field.link */
 	} window_switcher;