Window size/position is not remembered

[sethvargo]

Hey there! Thank you for an awesome project, I really love it. I noticed since 0.15.1964 dev (0.15.1964), gitx does not remember its previous window position or size after closed.

Steps to reproduce:

1. Install gitx utility via the command line
2. Run gitx . from a git repo
3. The window will open (for me it's always in the same place, the same size, and the internal panes (frames) are also in the same place
4. Resize and move the window and internal frames
5. Quit gitx
6. Run gitx . again from the command line
7. No changes are saved

I also put together a quick video illustrating the problem because it's hard to describe in words: http://recordit.co/8WYqw0u3GS

[sethvargo]

This might be a duplicate of #402