[kcmkwin/deco] Bring back KNewStuff support

KNewStuff is no longer hard-coded to Aurorae themes. Instead the
availability of KNewStuff is derived from the available plugin
metadata. If the section org.kde.kdecoration2 contains a key
"KNewStuff" it's value is interpreted as the knsrc config file name.
If there is at least one plugin with such a key KNS gets enabled.

If there are multiple plugins providing KNS support the download
button is turned into a button with a connected menu and each menu
entry points to one of the available resources. Of course this is
not optimal, but KNS doesn't allow the combining of multiple config
files.