This simplifies the plugin loading. Decorations just have to use
K_PLUGIN_FACTORY to specify how the KDecorationFactory needs to be
created. The KWIN_DECORATION macro is adjusted to generate the
boiler plate code, but it now needs to specify the name for the
pluginfactory and the KDecorationFactory.

This also transits the decoration abi version check to use
K_EXPORT_PLUGIN_VERSION which also simplifies the loading.

As a result the complete canLoad handling in DecorationPlugins is
removed.

REVIEW: 115930