Fixed texture memory leak when reloading textures after app resume on Android.
Removed TextureContent._bitmap.
It was only used in two locations:
1. Resizing a texture (for the resize-pow-two condition in TextureProcessor). It allocates a bitmap and then disposes it when done with this operation.
2. Font processor works with bitmaps, but rather than assuming there is one on the texturecontent up front, it just creates one.

This means that everyone else in the pipeline can rely on TextureContent.GetData/SetData always being what you expect it to be instead of being mysteriously out of sync with a _bitmap that your custom processor doesn't know about or have access to.
We now account for time stamp changes in the source file.
Merge pull request #2642 from SickheadGames/sourcetime

MGCB Source DateTime
The context menus for items and folders are now defined in the WinForms designer.
Reusing the existing add/remove item logic in the context menus.
Added per-item build/rebuild/clean options.
Further context menu cleanup.
We can now save temporary partial projects.
Added support for rebuilding individual content items.
When game is re-activated the mouse button state will reset itself to actual values.
Merge pull request #2644 from SickheadGames/FormMouseButton

MouseState.Button out of date
Fixed byte order used for colorkey and premultipled processing.
Fixing Mac build.
Merge pull request #2643 from SickheadGames/pipeline6

Pipeline Tool Item Rebuilds
Removed commented out code.
Merge pull request #2635 from SickheadGames/BitmapCleanup

Removed TextureContent._bitmap.
Add support for SamplerState.MaxMipLevel (WindowsGL and iOS)
Divided accelerometer values on Android by gravity to match XNA and iOS.
Merge pull request #2646 from flyingdevelopmentstudio/MaxMipLevel

Add support for SamplerState.MaxMipLevel (WindowsGL and iOS)
Merge pull request #2647 from Andreyul/accelerometer

Divided accelerometer values on Android by gravity to match XNA and iOS.
Merge pull request #2628 from Andreyul/delete_gl_texture

Fixed texture memory leak when reloading textures after app resume on Android