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IGraphicsContext.VSync property is obsolete, but we have to call it in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.
Make GLControl work on OS X

This isn’t the greatest solution, but at least it doesn’t require us to
reference WinForms in OpenTK and it works perfectly for what I’m using.
Fix potential exception that I was ignoring.

This can happen, so we should handle it
Update spacing and formatting to match existing style
Call the new code instead of leaving the old version untouched
Merge remote-tracking branch 'sappharad/develop' into develop
Merge remote-tracking branch 'frassle/warnings' into develop
Merge branch 'issue69' into develop