Refactor GUI to use ofxDatGui for all elements.

[damellis]

Thinking about it more, it seems likely that we're going to need more sophisticated event / focus handling for the GUI to work properly. For instance, we probably want a way to bubble events up from a specific GUI element up through its containing elements, as well as to trigger actions on focus gained and lost, etc. Rather than implementing all of this GUI logic ourselves, in an ad-hoc way, it seems like it's going to be easier in the long run to use the ofxDatGui framework, which has much of this logic.

This would be a relatively big change, and would include a strong dependencies on ofxDatGui. In particular, we'll need / want to make our ofxGrt and other plots inherit from ofxDatGuiComponent. We may also need to add a tab control to ofxDatGui.

This should probably be done in a branch, potentially even starting a few commits back (leaving the master branch at the state it was during the workshop).

@nebgnahz let's discuss this afternoon.