Fix gtk+2.0-maemo alternate subpackages

Created by: Decatf

This is to fix issue #1284 (closed). Specifying alternate subpackages in the provides variable of the main package means that the main package itself is providing an alternate for the subpackages. e.g. gtk+2.0-maemo provides an alternate gtk+2.0-dev.

To do it properly the provides variable must be overridden in each subpackage split function. I've compiled and tested deadbeef player with GTK2 UI and run it on Xfce4.

I haven't been able to properly test Hildon which this gtk+2.0-maemo package is intended for. Hildon doesn't work on my device. I am not 100% sure if the QEMU image is working right therefore am not sure about considering that as having tested this. The QEMU image boots and shows the Hildon background but the top left menu isn't super responsive. Nothing shows except the semi-transparent overlay. Can someone with a working Hildon setup test this patch?