[Feature] Separate settings from state (`preference.json`)

[andgeno]

The file in ~/.config/SourceGit/preference.json appears to contain user settings but also state info like this:

• LauncherPositionX
• LauncherPositionY
• WorkingCopyLeftWidth
• IsActive (which seems to flag the currently selected workspace)
• etc.

It would be great if state information could be stored in a separate file, e.g. state.json.

This would allow users to version their SourceGit settings without seeing constant changes that are not essential.