Add support for Android 9-patch images in BorderImage

This is required to have proper styling of Qt Quick Controls, and is a nice addition.

The idea would be to leave the API of that component type alone, but add the possibility of setting a source which is a .png containing border information. The component type would then have to be expanded to support all the features of the file format, but it would not be possible to do the same using code, at least not at the moment.