GbEditorView: add getter for GbEditorFrame 1 and 2
GbEditorDocument: reset progress before updating progress in callback
Tab, View: track changes from tabs to views.

This gets vim commands working again for instance.
build: bump libgit2-glib version

we were depending on this, it just wasn't fixed upstream yet.
icon: remove white from icon

looks bad, but we need to go to symbolic anyway
documents: add plumbing for save action via document stack.
editor: implement saving via document interface