MonoGame & FNA sample upgrade

[morfah]

It's good to dispose ImGuiRenderer when unloading content. For example if the game use multiple game states which unload and loads content.

Also if you instantiated ImGuiRenderer again there would be multiple event handlers for text inputs, so I removed those in the Dispose too.

Enabled docking in BeforeLayout, otherwise you would get an cimgui assert error when you tried to use the docking feature.

Did some minor code cleanups.