Auto fallback if savegame location is not writeable

[Brianum]

Implement a (platform-dependent) fallback location for savegame storage if the default location (working-dir) is not writeable.

For windows this could be %appdata%\EasyRPG(GameTitle)
For linux this could be ~/.config/EasyRPG/(GameTitle)
For macos this could be ~/Library/EasyRPG/(GameTitle)

Additionally Player could check for the presence of savegames created by RPG_RT.exe in %localappdata%\VirtualStore and use that path as a fallback location on windows.

This would make it much easier and less error prone to bundle the EasyRPG player with games.
(Example: Game installed to \Program Files or read-only MacOS Application bundle in /Applications)

[carstene1ns]

See also: #1928 (comment)