Make aptdaemon a soft dependency

On new Ubuntu versions (i.e. utopic) there's no aptdaemon.gtkwidgets anymore.
It was replaced by aptdaemon.gtk3widgets, which we cannot use since gtk3 and
gtk2 conflicts.

We should change aptpackageinstaller to use packagekit, but meanwhile lets not
make it a hard dependency.

Change-Id: Ibe89b6f2269f0af77c6b0de661df67aced69211e