Write a compatibility window controller section to the project file

[magnesj]

Alternative to #14601. Only one of the two should be merged.

Fixes #14600

#14601 solves the problem by making MdiWindowController the keyword written to file again, and keeping DockWindowController as a read alias. This PR keeps DockWindowController as the keyword of the window controller in use, and writes the main window id a second time using the old keyword, so the previous release reads MdiWindowController and the dev branch reads DockWindowController.

Both sections cannot live inside the same field. PdmFieldXmlCap<PdmChildField>::readFieldData reads a single start element and then does one readNext() expecting the end element of the field, so a second sibling element desynchronizes the stream. RimViewWindow therefore has two child fields:

<WindowController>                      <!-- read by 2026.06 and older -->
  <MdiWindowController>
    <MainWindowID>1</MainWindowID>
  </MdiWindowController>
</WindowController>
<DockWindow>                            <!-- read by the dev branch, unknown field to older versions -->
  <DockWindowController>
    <MainWindowID>1</MainWindowID>
    <ViewToControl>...</ViewToControl>
  </DockWindowController>
</DockWindow>

Unknown fields are skipped by PdmXmlObjectHandle::readFields, so the previous release ignores DockWindow without complaining.

The compatibility section cannot be empty. The old controller resolves its main window using mainWindowByID( m_mainWindowID ), and the default value -1 returns a null main window, so updateViewerWidget() returns early and the plot is still invisible. The main window id has to be written. The geometry fields can be left out, RimMdiWindowGeometry::isValid() returns false and the previous version falls back to a default subwindow size, the same path the 3D views already take.

Removing the compatibility section after 2-3 major releases is deleting one field and one class.