fromVariantMap() called twice on all plugins

[JulianThijssen]

@ChangLi27 noticed that when loading a project, for every plugin in the workspace their fromVariantMap() function is called twice.

We traced it back to a bug in the ViewPluginDockWidget.cpp

The last constructor is called, which in turn calls the first constructor via the ViewPluginDockWidget() in the initializer list. Both of these constructors add the widget to the active list, which is processed later when loading the workspace and calls fromVariantMap() on each of them.

The issue was fixed by removing the active << this; in the first constructor, but probably this is not the best solution.