move workspace/plasma/lib to workspace/lib/plasma so that we can do:

#include <plasma/foo.h>

in headers in libplasma. this is important so that they can be used post-install by third party plugins, apps, etc.

also, changed Applet::loadDataEngine to just Applet::dataEngine and make it actually return the data engine; move the applets to using it. safer and fewer LoC