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/feature.h

@@ -928,6 +928,13 @@
 # endif
 #endif
 
+/*
+ * +wayland_focus_steal	    Focus stealing support for Wayland clipboard
+ */
+#if !defined(FEAT_WAYLAND_CLIPBOARD) && defined(FEAT_WAYLAND_CLIPBOARD_FS)
+# undef FEAT_WAYLAND_CLIPBOARD_FS
+#endif
+
 /*
  * +dnd		Drag'n'drop support.  Always used for the GTK+ GUI.
  */