Proof of concept X-Poedit-Potfile header

Hello,

When using Poedit, I usually have projects with pot files.

With the "Update from POT File" I always have to navigate to the pot file (which is always in the same location for a project).

I decided I'd try a proof-of-concept X-Poedit-Potfile header storing the location of the pot file. See changes (I do not regularly program C...)

I've only tested this on MacOS X with a relative path (and no BaseDir set).

What's your opinion?