Add --disable-libunique configuration option for GTK3 builds. This builds a port from libunique to GtkApplication. keep GTK2 builds unchanged

Caja can now be build with GTK2 and libunique, GTK3 and libunique, or GTK3 without libunique using GtkApplication instead

GtkApplication port  Based on cherrypicked nautilus commits from
https://github.com/GNOME/nautilus/commit/a8481ee4bd8d34e792d63598fa5efb47736f9de4
main: adapt to GtkApplication changes
through
https://github.com/GNOME/nautilus/commit/c3382e0415d51082545f277c380d37be160e8d2d
application: move nautilus_application_new() to its own function

GTK3/GtkApplication builds:  add --force-desktop option
This is useful for other DE's

All: StartupNotify=false in .desktop files, as caja never connects to notification daemons and in GtkApplication builds this causes busy spinning curors

The button underallocation warnings came from pathbar sliders picking up padding from themes, then being forced to render to a smaller size in the code.

The GTK_ICON_LOOKUP_GENERIC_FALLBACK warnings came from the sidebar icons, this flag is now a source of errors. Unfortunately NULL does not work, so set GTK_ICON_LOOKUP_FORCE_SIZE, which does not create errors and ensures the icon size we actually want in the sidebar is used anyway.

Stops all but two of the warnings listed in
https://github.com/mate-desktop/caja/issues/561

Composited case still requires desktop, desktop window to be set transparent. Changes in last commit only affected noncomposited case

This reverts commit 8d658fa4f3790972c38e04df23eaa7b88b5066d8.

No need of this after latest commit.

The selectors to hold the desktop transparent in GTK 3.21 or later builds seem to be ignored in GTK 3.20 or older builds where Caja draws the desktop. Ideally some makefile work and two raw files would apply this only in GTK 3.21 and later builds