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/input/cursor.c

@@ -882,7 +882,7 @@ handle_motion(struct wl_listener *listener, void *data)
 	cursor_set_visible(seat, /* visible */ true);
 
 	if (seat->cursor_scroll_wheel_emulation) {
-		uint32_t orientation;
+		enum wl_pointer_axis orientation;
 		double delta;
 		if (fabs(event->delta_x) > fabs(event->delta_y)) {
 			orientation = WL_POINTER_AXIS_HORIZONTAL_SCROLL;
@@ -1014,7 +1014,7 @@ is_double_click(long double_click_speed, uint32_t button,
 		 */
 		last_button = 0;
 		last_view = NULL;
-		last_type = 0;
+		last_type = LAB_SSD_NONE;
 		return true;
 	}
 	return false;