gtkcssprovider: add clear_named_cache() function to cleanup cached providers
@3v1n0
Submitted by Marco Trevisan Link to original bug (#792851)
Description
In mutter, changes to a theme aren't applied to the frames until mutter it's reloaded. This is not really convenient and it's caused by the fact that the CSS providers aren't updated accordingly, nor could be recreated if named.
Thus the need to clear them. After some discussion I come up with just a clear() function that does it for all the providers, but it could be also changed to be per provider. As you wish.