patch 9.1.1784: Wayland code can be improved

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

This the second attempt to refactor the Wayland code base:
- Move clipboard code from 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: #18324

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

src/main.c

@@ -682,15 +682,18 @@ vim_main2(void)
     if (!gui.in_use)
 # endif
     {
-	if (wayland_init_client(wayland_display_name) == OK)
+	sprintf(wayland_vim_special_mime, "application/x-vim-instance-%ld",
+		mch_get_pid());
+
+	if (wayland_init_connection(wayland_display_name) == OK)
 	{
 	    TIME_MSG("connected to Wayland display");
 
 # ifdef FEAT_WAYLAND_CLIPBOARD
-	    if (wayland_cb_init((char*)p_wse) == OK)
+	    if (clip_init_wayland() == OK)
 		TIME_MSG("setup Wayland clipboard");
-	}
 # endif
+	}
     }
 #endif