Add additional enum value "AllButtons" to Qt::MouseButton (in qnamespace.h)

From IRC: Alan Alpert and I both find it awkward to 'OR' a large number of Qt::MouseButton values, when setting 'acceptedButtons' for a MouseArea. To simplify QML coding (for both typing and readability), we should provide a shorthand value for "allButtons". It should not include reserved bits- so the highest-order bit to include is MaxMouseButton, value 0x04000000.

AllButtons = 0x07ffffff.

It is simple to do this in the namespace, more complicated to add and maintain a new keyword (e.g., "acceptAllButtons = true) in the code. Considerably less documentation, as well.