When the decoration is reset a signal is emitted that the config
might have changed which the decoration can connect to for
reloading its configuration. For this an invokable method is
added to Aurorae allowing to read a config value which just
returns the QVariant.

Proper support for border sizes are added by providing the enum
in DecorationOptions, so that QML themes can use the enum values
to decide which border size to use.

The kcm is adjusted to also support these config mechanisms and
to properly load and save the border sizes for QML based themes.