Extract Effect into its own header

After merging libkwineffects and libkwin, EffectsHandler and EffectWindow
have effectively become redundant. On the other hand, Effect and the
associated factory code is still relevant. In order to split relevant and
"legacy" code, this change extracts the Effect class in its own header.
It's also a good idea to split kwineffects.h header because it's quite
huge...