Provide type checking for initial startup state provided by hosting app

[gideonthomas]

We have a bunch of state that we pass in via the hosting app or set defaults in the StartupState, particularly for UI preferences https://github.com/mozilla/brackets/blob/master/src/bramble/StartupState.js#L15. Here: https://github.com/mozilla/brackets/blob/master/src/extensions/default/bramble/main.js#L185-L195, we initialize that state blindly without checking if the values are being set to what is being expected (for e.g. booleans, objects, etc.). This results in us having to do some pretty heavy-handed type checking here: https://github.com/mozilla/brackets/blob/master/src/extensions/default/bramble/lib/UI.js#L89-L110. Let's try to move that type checking into the .ui function so that we can define properties on it and specify the types they accept. I think the way PreferencesManager does it is pretty nice (https://github.com/mozilla/brackets/blob/master/src/editor/Editor.js#L159-L222) and might be a model we want to follow.