Improve finding of plugins by id

Rather than listing all plugins in the given namespace and searching
through their metadata, the pluginId is saved during the registration
and we can search through it in logarithmic time.
Most importantly, we do not need to reparse the JSON. Caching it would
also be possible, but depending on the usecase it might depend in
unneeded memory being allocated.

Using a QHash instead of QMultiHash increases performance of the normal plugin searches too.