Implements T7243
Not too fond of this "Current" (carried over from the old code). Perhaps we could do some magic to only show it when the current theme is not in the list or is a modified version of a theme? That would make the dialog less confusing as to what "Current" means.
{F5810572}
TODO:
* Make applying theme work (write config and signal applications)
* Make restore defaults work
* Make "Install from file" work
* Make edit work (the edit dialog is quite entangled with the code, I thought it was completely standalone nowadays)
* Make removing theme work
* Make the "apply to non Qt apps" checkbox work (no idea why anyone would want to uncheck this, though)