Add an option to pick light/dark icons regardless of theme

[helpingpigeon]

Now that OBS has native GTK support (great!), my dark theme doesn't look very good with the dark icons the System theme uses. It might be nice to have an option for that.

In the meanwhile, I modified the System theme to use icons from the dark theme as a workaround. It's in the attachments (or here: https://pastebin.com/raw/CDZXHQbh).

Just put the .qss file in /usr/share/obs/obs-studio/themes.

 

[helpingpigeon]

Updated to work with the redesign and include the latest icons.

Pastebin: https://pastebin.com/raw/BZMYwjr2