Ensure binary effects are loaded before cleints

The current code performed two functions:
 - metadata was read async
 - the event loop is spun between each effect loading

After we merged static and binary plugins this caused a problem for
things like blur and background contrast where loading an effect
influences what we advertise as supported. We don't want plasmashell to
have to reload all it's SVGs.

All KDE provided binary plugins are statically bundled so querying for
plugins is super duper fast anyway. It's not the same kbuildsycococa
path that it used to be.

The C++ plugins tend to be considerably faster in their constructor than
scripted counterparts, and if anything should be loaded lazily it can be
handled inside the effect.