[Gtk] Borders, Background, Shadow: refactor to use Widget.Drawn

[lytico]

issue

1. Border support is limited with Gtk.Css-Features.

2. There is a problem with Drawing a Picture as Background

Currently this is implemented with an url to a tempfile

3. Shadows doesn't work at all, although implemented:

ViewExtensions.UpdateShadow

proposal

Draw borders / backgrounds using Widget.Drawn-Event

set in ViewHandler / Borderhandler

and draw Colors/Paints/Borders over Cairo.Context