Simplify provision of filename for environment{Init,Save}

[johanmalm]

...to avoid having to working it out against in maindialog.cpp ::onApply() method. Simply just remember the filename in the environment.cpp translation unit.

@stefonarch Are you happy carry on doing some independent peer-reviewing? Just say if it gets too much.

[stefonarch]

You overestimate my skills ;)
I just got an understanding how the read/write mechanism works.
I can test it, that's all.