Fix missing dirSeparator & print ini file locations:

Clicking "apply" or "ok" in the options menu created a file
~/OpenFlipperOpenFlipper.ini

When fixing this, I also lightly refactored the code to remove
duplicate filename computation.

There still are things left to do for the next person:
    - Factor out remaining duplicate code - why is there an extra
      function just for keybindings?
    - Figure out how that global variable containing the list
      of option files should be used, or if we really want it.
    - Find out why we still have multiple local configfiles