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/view.h

@@ -274,7 +274,7 @@ struct view_query {
 	struct wl_list link;
 	char *identifier;
 	char *title;
-	int window_type;
+	enum lab_window_type window_type;
 	char *sandbox_engine;
 	char *sandbox_app_id;
 	enum lab_tristate shaded;