Refactor `Panels` so they can be reused and rearranged.

Previously, several Panel classes derived directly from QDockWidget, so they could be docked by the user in whatever arrangement was desired.

To allow re-use and rearrangement, these Panel classes now derive from QWidget, and are placed inside a pqSMTKDock which derives from QDockWidget. pqSMTKDock has a template parameter to allow it to create the child Panel of the correct type. Panel classes must now implement void setTitle(QString title) to provide the pqSMTKDock with the correct title, and use setWindowTitle() to provide the initial dock window title.