Add option to make login effect fade via black instead of a long transition to your desktop.

Nuno has to decide whether the new option will become default or not, for now it is disabled by default.
You can enable it in effects kcm, now Login effect has a configuration dialog.

It does not (yet) however check for non-fullscreen effects, so they might look weird when this is enabled.

REVIEW: 101812


C +22 -17 kwin/effects/login/login_config.h [from: kwin/effects/login/login.h - 057% similarity] [License: GPL]