Added File > Import in which you choose a .contentproj file.
File and Build menu items now only appear as enabled when appropriate (eg, no save option if project is not dirty).
Project will now load even with broken references.
ContentItems with importers or processors which we fail to locate will now be shown as "missing".
Changing project references will now trigger content items to re-resolve their processor/importer type from the specified name.
Initial support for ANGLE platform
Fixed OpenAL and OpenTKGameWindow on ANGLE
Ignore MonoGame.Framework.Angle.* files
Add Protobuild target for ANGLE
Fixed GraphicsDevice init on ANGLE
Support SoundEffect instances on ANGLE
Fixed compilation on MonoMac
Fixed compilation on MonoMac (part 2)
Fixed compilation on Windows

Also removed an unnecessary MonoMac-specific codepath (the code was
effectively identical to the regular OpenTK codepath.)
Revert "Fixed compilation on Windows"

This reverts commit ba07b24bf4459dccf5f2ff3ec58233c63071aae9.
Fixed compilation on Windows

Without modifying working code
Only implement occlusion query on ANGLE

This requires more testing before it can be enabled on iOS and Android.
Fixed compilation on Windows (part 2)
Fixed incorrect inclusion of Desktop/Input/GamePad.cs
Fixed compilation after rebase
Request OpenGL ES 2.0 context on ANGLE

This needed to be rewritten after the recent rebase. Context
construction is now more robust, too.
Potential fix for WindowsPhone build error
Added documentation for classes in Microsoft.Xna.Framework.Audio
Fix runtime error on Mac platform when setting the Title.  Was throwing NoImplemented exception because Title is checking if Window is null or not.  If it is still null at this point it will throw that error.  Moving it to OnLoad works.