BarDescriptorEditor: Create widget from editor

Not the other way round. Results in less, and less convoluted code.

Change-Id: I58a8d1e7785c0d712ff518d38425fc5646a262dd
Reviewed-by: Tobias Nätterlund <tobias.naetterlund@kdab.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>