ENH: Allow loading modules [from Python]

Move qSlicerAbstractModuleFactoryManager::registerModule(QFileInfo) from
protected to public Q_INVOKABLE, so that users (Python users in
particular) can manually register additional modules at run time. Add
new (also public Q_INVOKABLE) method to qSlicerModuleFactoryManager
allowing users to request that a specific list of modules be loaded (and
instantiated, if necessary).

This allows e.g. a user at the Python console to dynamically load a
module that is not in the search paths at run time, without modifying
the search paths. In particular, this will be useful to the Extension
Wizard module.