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.

src/img/img-xbm.c

@@ -251,7 +251,7 @@ parse_xbm_builtin(const char *button, int size, uint32_t color)
 		t[i].value = button[i];
 		t[i].type = TOKEN_INT;
 	}
-	t[size].type = 0;
+	t[size].type = TOKEN_NONE;
 	process_bytes(&pixmap, t, color);
 	return pixmap;
 }