patch 9.1.1725: Wayland code can be improved

Problem:  Wayland code can be improved
Solution: Refactor Wayland Clipboard code (Foxe Chen)

This refactor makes the Wayland codebase less convoluted:
- Move clipboard code in wayland.c to clipboard.c
- Use C99 bool type
- Properly poll the Wayland display file descriptor
- Instead of checking if the data source is not NULL in order to
  determine if a selection event comes from us, use a special mime type to
  identify selection events coming from ourselves. The problem with the
  previous approach is that race conditions may occur.
- Put the focus stealing code under a new feature "wayland_focus_steal"
- Use ELAPSED_* macros instead of gettimeofday()
- Pass tests
- Reimplement commented out code
- Update docs
- Make Wayland clipboard behaviour more in line with X11 when connection is lost
- add missing malloc checks and possible memory leaks + refactored some
  tests.

closes: #18139

Signed-off-by: Foxe Chen <chen.foxe@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/structs.h

@@ -5237,20 +5237,26 @@ struct cellsize {
 
 #ifdef FEAT_WAYLAND
 
+typedef struct vwl_connection_S vwl_connection_T;
+typedef struct vwl_seat_S vwl_seat_T;
+
+# ifdef FEAT_WAYLAND_CLIPBOARD
+
+typedef struct vwl_data_offer_S vwl_data_offer_T;
+typedef struct vwl_data_source_S vwl_data_source_T;
+typedef struct vwl_data_device_S vwl_data_device_T;
+typedef struct vwl_data_device_manager_S vwl_data_device_manager_T;
+
+typedef struct vwl_data_device_listener_S vwl_data_device_listener_T;
+typedef struct vwl_data_source_listener_S vwl_data_source_listener_T;
+typedef struct vwl_data_offer_listener_S vwl_data_offer_listener_T;
+
 // Wayland selections
 typedef enum {
-    WAYLAND_SELECTION_NONE	    = 0x0,
-    WAYLAND_SELECTION_REGULAR	    = 0x1,
-    WAYLAND_SELECTION_PRIMARY	    = 0x2,
+    WAYLAND_SELECTION_NONE	= 0,
+    WAYLAND_SELECTION_REGULAR	= 1 << 0,
+    WAYLAND_SELECTION_PRIMARY	= 1 << 1,
 } wayland_selection_T;
 
-// Callback when another client wants us to send data to them
-typedef void (*wayland_cb_send_data_func_T)(
-	const char *mime_type,
-	int fd,
-	wayland_selection_T type);
-
-// Callback when the selection is lost (data source object overwritten)
-typedef void (*wayland_cb_selection_cancelled_func_T)(wayland_selection_T type);
-
-#endif // FEAT_WAYLAND
+# endif
+#endif