Add option to save config.arm in $appdata

Whenever I update, I forget to not update the config.arm.

I'd prefer, if I had a chance to tick a box to have it save my config.arm in an $appdata folder, and whenever I start Armorpaint, it checks if one is available, and if yes, loads my configs from there.

If not, or if the option is not ticked, then load one from the armorpaint/data directory.
I know it might be finicky to have a good solution, but I think this might come in handy later down the road.

Start by locating where config.arm is loaded and saved, where startup checks the armorpaint/data directory, and where preferences or options are defined. Trace the existing configuration flow before deciding how the $appdata location and checkbox should fit; done means the selected setting persists config.arm there and startup falls back to armorpaint/data when appropriate.

Written by the indexing model from the issue text.