Abstract event handler in UIContext
Fix crash resulting when using nom::hint

* This would occur when we'd return an unsuccessful result from
SDL_GetHint.
Add a hint for quickly testing emulated SDL2 logical viewport

* RocketSDL2Renderer: Add corrective clipping calculations using
viewport dimensions for GL clipping method within ::SetScissorRegion.
Introduce handling of window resize events for libRocket contexts

* Introduce nom::UIContext::load_font, nom::UIContext::resize.
Rename RocketSDL2Renderer to RocketSDL2RenderInterface
Fix memory management issues

Do not manage pointers with shared_ptr; documents must not have their
references removed upon destruction -- the context internally does this
for us (libRocket).

* Introduce UIWidget::close method.
* Introduce void default ctor for UIMessageBox; this allows us to use
the class as an object reference more easily.
Texture: Change err to warning regarding Scale2X unavailability
UIMessageBox: Bit of clean up on alignment methods
Move UIWidget class to its own files