implements https://github.com/gitextensions/gitextensions/pull/1505#issuecomment-11919305.

- help image visibility can be toggled
- Remember help image visibility per branch. This is currently working only for the current GitExt session. But can easily be persisted to disk (see private void SaveIsExandedValueInSettings(bool value) and private bool LoadIsExandedValueFromSettings(bool defaultValue))