Simplify PluginManager::instance()

This simplifies the PluginManager::instance() method by using
a static singleton and removing the QMutex.

Change-Id: I4eed0513d1e4cf523bd6093f9c6761e3e6feb73e
Simplify plugin loading in MongoChem::MainWindow

This simplifies the factory plugin loading in MongoChem::MainWindow
by removing duplicated code and using a single generic method.

Change-Id: Ia6660daaf017ac0224d9425a8a62fa5d07cacd8f