[sensorfw] Use Q_OBJECT on all plugin classes to make them loadable.

moc needs Q_OBJECT before it parses Q_PLUGIN_METADATA, so this never worked.
[sensorfw] Fix plugin loading under Qt 5.

The plugins reside in a different directory, so we need to take that into
account. Also remove a number of bizarre hacks that seem to have been designed
to work around providing a full path to the plugin. It's more efficient to use a
full path.