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/wayland.txt

@@ -1,4 +1,4 @@
-*wayland.txt*   For Vim version 9.1.  Last change: 2025 Aug 27
+*wayland.txt*   For Vim version 9.1.  Last change: 2025 Sep 01
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -105,7 +105,8 @@ To solve this problem, Vim implements a way of gaining focus in order to
 access the clipboard, by creating a temporary transparent top-level surface.
 This is by default disabled and can be enabled via the 'wlsteal' option.
 Moreover, a seat that has a keyboard is also required, see 'wlseat', and the
-xdg-shell protocol must be available.
+xdg-shell protocol must be available.  Additionally, Vim must be compiled with
+the |+wayland_focus_steal| feature.
 
 Note that this method can have several side effects from the result of focus
 stealing.  For example, if you have a taskbar that shows currently opened apps