Allow change M210Projects config folder location (to same folder with BuildGDX.jar, or choose a folder)

Hello.

Right now (at least <1.17), the config path for BuildGDX is:

• Windows: C:\Users\<username>\M210Projects\ or %USERPROFILE%\M210Projects\.
• Linux: /home/<username>/.m210projects/ or ~/.m210projects/ (related issue #99).

The improvement is something to override this and allow to choose a path. Or at least, a way to make the M210Projects alongside the BuildGDX.jar file and use the same path as that file.

The reason for this is if you are using multiple partition/disks, and you have BuildGDX in one path, I want the save files near it to save the save data/files and store it together, instead of having them hidden in Windows/Linux folder. Normally what i do is create a symbolic link between the Windows folder and link it to a folder I created alongside BuildGDX.jar.

Thanks!