* Core/Utils: Use correct env for save_filename from AppImage
* OptionsCreator: run export on a separate thread
Running a blocking call from kivy misbehaves on Linux.
This also changes '*.yaml' to '.yaml' for Utils.save_filename,
which is the correct way to call it.
* Core/Utils: destroy Tk root after save/open_filename
This allows using those functions from multiple threads.
Note that pure Tk apps should not use those functions from Utils.
* OptionsCreator: show snack when save_filename fails
* OptionsCreator: disable window while exporting
* OptionsCreator: fixing typing of added stuff
