Revert "Do not use gtk_icon_theme_load_icon() as it's not thread safe"

The following commit fixes the threadsafety issue in another way.

This reverts commit 6a3215403d04cfc99efbb5e04c8e84670d012342.

Conflicts:
	src/gs-utils.c
Fix threadsafety issues with gtk icon theme use

Instead of using global icon theme, create our own icon theme singleton
and use that. Having our own makes it easy to serialize access to it
when we need to load icons from several threads at a time.