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/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Aug 24
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Sep 01
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -13161,7 +13161,9 @@ vreplace		Compiled with |gR| and |gr| commands. (always true)
 vtp			Compiled for vcon support |+vtp| (check vcon to find
 			out if it works in the current console).
 wayland			Compiled with Wayland protocol support.
-wayland_clipboard	Compiled with support for Wayland selections/clipboard
+wayland_clipboard	Compiled with support for Wayland clipboard.
+wayland_focus_steal	Compiled with support for Wayland clipboard focus
+			stealing.
 wildignore		Compiled with 'wildignore' option.
 wildmenu		Compiled with 'wildmenu' option.
 win16			old version for MS-Windows 3.1 (always false)