Continuous build

GTK4: Convert Geeqie to a GtkApplication

GTK4 migration

The following options are deleted:
--fullscreen-start, --fullscreen-stop: Use --fullscreen which is a
toggle
--slideshow-start, --slideshow-stop: Use --slideshow which is a toggle
--tools-show, --tools-hide: Use --tools which is a toggle
--remote: No longer needed
--disable-clutter: Start with GQ_DISABLE_CLUTTER=y[es] geeqie
--cache-maintenance: Start with GQ_CACHE_MAINTENANCE=y[es] geeqie
(disabled in this version)
--new-instance: Start with GQ_NEW_INSTANCE=y[es] geeqie
--blank, --list-add, --list-clear: I do not see the need for them

GQ_CACHE_MAINTENANCE=y geeqie is disabled. It will be fixed in a future
commit.