Add method to allow user to request a plugin by identfier.

Add pluginFactory(const QString &id) to lookup
a plugin factory by name.

Change-Id: I9e6c4b3d8e7548673c063a408936e4735595c59c
Merge topic 'plugin-by-name' into master

22ba8791 Add method to allow user to request a plugin by identfier.