Preserve dock parents to avoid black QOpenGLWidget views

[kriben]

Switching dock tabs reparents inactive non-native widgets to nullptr, temporarily turning them into top-level windows. For QOpenGLWidget children, these transitions can invalidate Qt composition resources even when shared OpenGL contexts remain alive, leading to black frames or views that recover only after another repaint.

The internalWinId() condition restricted parent preservation to dock widgets with an existing native window handle. A non-native dock can also contain a QOpenGLWidget, which normally renders into an offscreen framebuffer that Qt composites into the containing window. Such docks need protection from temporary top-level transitions too, so native-window status is not an appropriate condition for preserving their parent.

Remove the internalWinId() condition and keep detached widgets parented to the dock manager regardless of whether they are native. This neither creates nor removes native window handles. Explicitly hide detached widgets before reparenting because setParent() may be a no-op. Retain the null-parent fallback when no dock manager is available and update the layout ownership documentation.

This avoids forcing native windows or adding extra repaint requests. No application-specific environment variable is required.

Related ResInsight issue: OPM/ResInsight#14708

[kriben]

Tested on Windows 11, Ubuntu 26.04 (Wayland) and Redhat 8 (X11): seems to solve the problem everywhere.