Remember splitter/panel sizes between sessions in history view

[inkvizytor]

Description

In the history view, the splitter positions between the commit graph and the commit details panel are not persisted.
Every time I restart SourceGit the panels reset to their default sizes, and I have to drag the splitters back to my preferred proportions.

I'm not sure whether this is a bug or simply a missing feature, but persisting these sizes would be a great quality-of-life improvement.

Steps to reproduce

1. Open a repository and go to the Histories view.
2. Resize the splitter between the commit graph and the commit
   details/changes panel (in either vertical or horizontal layout).
3. Close and reopen SourceGit (or reopen the repository).
4. The splitter is back at its default position.

Expected behavior

Splitter/panel sizes are remembered per layout mode (vertical and horizontal separately, since the proportions that make sense differ between them) and restored on the next launch.