Fix compile warnings in Examples.

This commit fixes every current warning in the Example code bar two.
Both due to the use of the OpenTK.GameWindow.Joysticks.
Fix compile warnings in Converter.
Fix XML documentation.
Disable Obsolete warning.

IGraphicsContext.VSync property is obsolete, but we have to call it in
the GraphicsContext.VSync implementation, so disable the obsolete
warning for this one use.
ModalLoopCallback is never assigned.

ModalLoopCallback is never assigned, so always has it's default value
null. Replace all uses of ModalLoopCallback with null.
Remove unused fields and variables.
Merge remote-tracking branch 'frassle/warnings' into develop