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>

runtime/optwin.vim

@@ -1,7 +1,7 @@
 " These commands create the option window.
 "
 " Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2025 Sep 20
+" Last Change:	2025 Sep 22
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 " If there already is an option window, jump to that one.
@@ -822,7 +822,7 @@ if has('wayland')
   call <SID>AddOption("wlseat", gettext("Wayland seat to use"))
   call <SID>OptionG("wse", &wse)
 endif
-if has("wayland_clipboard")
+if has("wayland_focus_steal")
   call <SID>AddOption("wlsteal", gettext("Enable wayland focus stealing functionality in order to access the clipboard"))
   call <SID>BinOptionG("wst", &wst)
 endif