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/output.h

@@ -3,8 +3,7 @@
 #define LABWC_OUTPUT_H
 
 #include <wlr/types/wlr_output.h>
-
-enum lab_edge;
+#include "common/edge.h"
 
 #define LAB_NR_LAYERS (4)