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>

runtime/doc/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Aug 27
+*options.txt*	For Vim version 9.1.  Last change: 2025 Sep 01
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -10195,7 +10195,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 				*'wlsteal'* *'wst'* *'nowlsteal'* *'nowst'*
 'wlsteal' 'wst'		boolean  (default off)
 			global
-			{only when the |+wayland_clipboard| feature is included}
+			{only when the |+wayland_focus_steal| feature is
+			included}
 	When enabled, then allow Vim to steal focus by creating a temporary
 	surface, in order to access the clipboard.  For more information see
 	|wayland-focus-steal|.