Remove NRE when changing ShowCheckBoxes

[mikkelbu]

Fixes #52

Note, I've decided not to update the setting TestTreeSettings.ShowCheckBoxes when changing Show CheckBoxes via the menu or via the context menu. This can be done via Model, if we choose so. The reason for this, is it seems that most changes in menus do not update the "default" settings in Tools => Settings (except for Full GUI vs Mini GUI).

Also note that we do not change the user interface, if we change the setting TestTreeSettings.ShowCheckBoxes. This will first have effect when we restart the program. We could do this by commenting in the method UserSettings_Changed (and do some more changes), but it does not seem like we update the UI immediately when changing settings in Tools => Settings.

So before making the above mentioned changes I think we should decide upon the connection between the settings in Tools => Settings and settings we can set directly in the GUI (in menus and context-menues). I think of them as the "default" vs. the "current" settings, but it can be that this is just my interpretation.

[CharliePoole]

@mikkelbu I agree with both what you have done and what you suggest we need to look into. I have started writing a sort of spec about how this should work and which settings take precedence.

Since this project is a look-alike of the old NUnit Gui, I often run that in order to see what it does in a particular case.

Anyway, merging this and we can change more in a follow-up PR. I'm working on a more general one to do with settings organization right now.