This brings back the configuration for decoration plugins. As a change
to the old variant the configure button is moved into the list view
together with the preview. It is enabled/disabled depending on data
provided by the DecorationModel. For a plugin the DecorationModel
queries for a boolean "kcmodule" key in the metadata. For a theme it
invokes the slot hasConfiguration with the theme name which returns
whether the theme provides configuration.

The actual opening of the configuration is triggered from the
PreviewBridge, which uses the existing KPluginFactory to load the
KCModule. The decoration plugin must provide the keyword "kcmodule"
for it.

So far Aurorae is adjusted and provides configuration for the Plastik
decoration. The interaction with the configuration module works, but
the configuration itself for Plastik seems to be currently broken.