Improve the preview mode

This is a continuation of !1845 (closed)

We added "preview" mode to ParaView recently. This MR makes several improvements to it.

1. It was noticed that "Preview" mode doesn't reset when the session is reset (Edit | Reset Session) (see discussion on #17273 (closed)). To support this case (and others such as lack of support for undo/redo, save/restore xml state, Python trace, or Python API for preview mode), this MR makes preview mode settable using a property on the view layout. This ensures that preview mode can work well with undo/redo, save/load state etc.
2. Now we are able to enter (if we are not already in) the preview mode automatically when saving screenshot or animation i.e. the UI updates similarly.
3. If separator width/color is changed in the save screenshot/animation dialogs, the layout changes to reflect the selected width and color for the view separator (Fixes #17554 (closed)).
4. If already in Preview mode when use does a "Save Screenshot/Animation", the dialog it init-ed to use the resolution picked for preview as well as toggle "Save All Views", if applicable.
5. Cleans up pqSplitter, so that is supports rendering a handle of pixel width 0 (simplifies code in pqMultiViewWidget).
6. Entering Preview mode no longer prohibits user from creating new tabs or changing active tab. Preview mode only affects the tab that was active when entering the preview mode, however.
7. Preview mode works even when a layout is popped out (fixes #17558 (closed)).
8. Also fixes other discrepancies with empty views or mismatching image resolution (Fixes #17651 (closed), #17843 (closed) ).