Fixes https://github.com/mate-desktop/caja/issues/688

*Split caja.css. Much of it can load with GTK_STYLE_PROVIDER_PRIORITY_THEME to allow themes that support Caja to set their own style. CSS required for the desktop background to show and to keep the white text suitable for most backgrounds needs GTK_STYLE_PROVIDER_PRIORITY_APPLICATION so it is loaded separately from new file caja-desktop.css

*Move all css items that must not be overridden by themes (most of them are for the desktop) to caja-desktop.css and keep those that are only needed when themes do not set them at all in caja.css

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