Allow Global Theme to set GTK2/3 themes

This Merge Request gives Global Themes the ability to set GTK2/3 themes on top of everything they can currently set.

This is upstreamed from Feren OS, with more Feren OS specific additions being planned in the near future.

For developers to use this feature, add this to the Global Themes' defaults file(s):

[GTK][Settings]
ThemeName=yourgtkthemehere

For example, with Breeze you'd add this:

[GTK][Settings]
ThemeName=Breeze

Preview (potato quality): Peek_2020-10-25_02-18 Ignore the GTK theme changer not applying the theme in Nemo - it did work when I wasn't recording, and I'm assuming everything else was just issues relating to being on KDE neon Git Unstable.