Updated submodule ThirdParty/Libs.
Added portable class library (PCL) MonoGame project.

This is the first experimental version.
Minimal changes using "#if PORTABLE" defines and throwing NotImplementedExceptions in PCL project.
This PCL DLL can be used for bait-and-switch PCL.
Added strong-name signing to WP8 project.

(If PCL DLL is strongly named, all bait-and-switch DLLs must be strongly named.)
Added missing GamePad.GetState overload.
Added Texture2D.SaveAsImage() for WINDOWS.

WINDOWS the existing MONOMAC code path.
WindowsPhone touch input also sets mouse input.

This is the same behavior as XNA/WP7, and allows existing mouse-based GUI code to run on WP without changes.
Fixed wrongly scaled touch position in Win8, and touch input also sets mouse input.

Touch input position was wrong if the back-buffer resolution was different from the window resolution.
Touch input now sets mouse position. (See also last commit for WP8).
Added properties Mouse.IsRelative property and MouseState.DeltaX/Y.

In XNA relative mouse input is usually created by resetting the mouse position in each frame. This method is not accurate for high framerates because a fraction of the mouse movement is lost. This method also does not work if Mouse.SetPosition() does not work.
The correct, accurate solution for relative mouse input in Win/DX is using the raw mouse device. On Win8 it is the OnMouseMoved event.
The new code also works when the old method (calling SetPosition() each frame) is used on other platforms.
Disabled creation of class documentation XML file in Win8 project to remove warnings.