MUI type, name, and namespace refactoring

Graphics.UI.GLFW should no longer need to be imported separately from
SOE (SOE exports everything necessary).
SOE.Event is now SOE.UIEvent to prevent namespace issues.
UIMonad.Input no longer exists (it was just a wrapper for SOE.UIEvent,
so we just use UIEvent now).
Some widgets have been refactored to be more stylistically consistent.
Added MidiWidgets

Moved all of the MIDI stuff from MUI.Widget to a new file
MUI.MidiWidgets.  Now, the core UISF stuff (SOE, UIMonad, UISF, Widget)
are basically MIDI free.