[Win] Refactor huge wndproc into functions
[Win] Remove unnecessary #if clauses
[Win] Do not unregister class twice

Only the parent window would register a class, but both the parent and
the child window would unregister it. This is now fixed.
[Win] Do not overload internal SetWindowLong

The internal function is now appended with "Internal".
[Win] Fix issue #33; fix issue #34

This patch adds a workaround for WM_STYLECHANGED messages that are not
delivered when running on Mono/Windows. If we detect Mono, then we call
HandleStyleChanged() directly in order to update the internal state of
our WinGLNative instance.
Merge pull request #38 from opentk/issue34

Fix for issue #33 and issue #34