Support for custom configuration dialogs for QML decorations

If the decoration package contains a ui file in ui/config.ui and
and KConfigXT xml file in config/main.xml the configure dialog
is extended by the custom values.

For this the ui file is loaded and included in the dialog's
layout. To make this possible the AuroraeConfigForm is changed
to a VBoxLayout containing the so far used FormLayout.

The configuration settings are loaded using Plasma's ConfigLoader
and attached to the widget through a KConfigDialogManager.

Marking this commit as the one which implements QML support for
Aurorae themes:

REVIEW: 105751
REVIEW: 105768
BUG: 303810
FIXED-IN: 4.10