Mouse input now matches XNA on WinGL and Linux

Bugfix for issue #2464
- Adds support for XButton1 and XButton2
- Correctly retrieves the desktop cursor position and translates
that to client coordinates
Updated to OpenTK 1.1.2
Removed platform-specific hacks

OpenTK 1.1.2 can change the hardware mouse cursor directly, so
take advantage of that to remove the dependence on WinForms and
platform-specific hacks.
Use SetMouseVisible to hide the cursor by default
Removed EditorBrowsable and fixed PrimaryWindow
Merge pull request #2583 from thefiddler/mousefix

Improve mouse input on WindowsGL and Linux