x11: Ignore old XFixesSelectionNotify events

GtkEntryCompletion can rapidly release and claim ownership of the
primary selection. This generates multiple XFixesSelectionNotify events,
first stating that no one owns the selection, then another stating that
we own the selection. The notification that no one owns the selection
causes GtkEntryCompletion to deselect the text, breaking inline
autocompletion.

This fixes it by ignoring any XFixesSelectionNotify with a timestamp
earlier than our clipboard timestamp.

Fixes #14
4 jobs for xfixes-timestamp in 25 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #68138
fedora-x86_64

00:05:54

passed #68139
win32
msys2-mingw32

00:08:58

 
  Flatpak
passed #68140
flatpak:demo

00:05:33

passed #68141
flatpak:widget-factory

00:05:18