Keep MdiWindowController as project file keyword

[magnesj]

Fixes #14600

The MDI to dock widget refactoring renamed the PDM class RimMdiWindowController to RimDockWindowController, and with it the class keyword written to the project file. In CAF_PDM_XML_SOURCE_INIT the first keyword is the one written to file, the remaining ones are read aliases, so dev started writing <WindowController><DockWindowController>.

Previous versions of ResInsight have no factory entry for DockWindowController. When PdmChildField::readFieldData meets an unknown class name it only fails when the field is empty at read time, and this is exactly what separates 3D views from plots:

• Rim3dView creates the controller in its constructor, so the field is already populated. The keyword mismatch only makes the stored fields be skipped, and the view is still displayed.
• Plots get a controller only from the project file. The factory returns null, the element is skipped, isMdiWindow() is false and no MDI subwindow is ever created, so the plot is silently invisible.

This restores MdiWindowController as the keyword written to file and keeps DockWindowController as a read alias, so project files written by intermediate dev builds still load. The dropped geometry fields are harmless, RimMdiWindowGeometry::isValid() returns false and the previous version falls back to a default subwindow size, which is the same path the 3D views already take.