logic to load Plasmoids

- add a mousearea to the delegate that calls
widgetExplorer.addApplet(pluginName)
- in widgetsexplorer, locate the plasmoid, load it via
Applet::loadPlasmoid(), then add it to the containment
- In showWidgetExplorer, we set the widgetExplorer's containment to the
sender(), if there's no containment set, we can't add the applet. This
way, the widgetexplorer knows which containment the applet should go to

This makes it possible to add Plasmoids via the GUI :-)