Turn KImageCache into a mixin for KSharedDataCache

Explanations needed on that one. :-)

We want KImageCache in KGuiAddons, but we can't depend on KCoreAddons
from there. Unfortunately KSharedDataCache (base class of KImageCache)
is in KCoreAddons.
We escape that unfortunate situation thanks to several moves:
* factor out the local cache code in its own class;
* introduce a template mixin class which inherits from the local cache
class and introduce all the KSharedDataCache dependent code, this
dependency will be resolved at compile time in the client code
(remember it's a mixin);
* to make it nicer to use have KImageCache as a #define of the mixin
instanciated for KSharedDataCache