Multi Page issues
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#### Summary:
- **all the bugs and issues found in multi-page feature in one place.**
1) the resize handle bug is back.
- **Info.** : resize handle is not updated when we change the page size.
- **steps to re-produce**
1) try to change the page size using the dropdown.
2) as soon as you change the page size, the page resize handle doesn't change it's place according to the new page size. (after changing the page size, when we zoom-in/out, it fix the handle position.)
- **note** : this only happens in **Single Page Document** mode.
- **video** :

2) ***Resize page to selection (Ctrl+Shift+R)*** doesn't respect the ***Move overlapping object as page is moved*** setting.
- **Info.** : when we try to do a **resize page to selection** on a object that is outside of the page, the page with objects on it is resized to that object which is outside of the page despite the fact that, **Move overlapping object as page is moved** setting is enabled.
- **steps to re-produce**
1) create 2-3 pages with objects.
2) now, create one object outside of the pages.
3) make sure the setting **Move overlapping object as page is moved** is enabled.
4) now, select the object that is out side of the page. (don't deselect the object. just click on that object and click on page tool)
5) now, click on page tool and do **Resize page to selection**
6) the page is resized to that object which is outside of the object even we've **Move overlapping object as page is moved** setting is enabled.
- **note** : this also happens when we select the objects of other pages. not just objects which is outside of the pages.
- **video** :

3) white viewbox bug.
- **Info.** : new white viewbox is created after doing the **Resize page to the selection** on a first page.
- **step to reproduce**
1) create 2-3 pages with objects.
2) now select any object and perform the **Resize page to the selection** operation on a first page.
3) now, create the page by mouse.
4) as soon as you create the new page, the white viewbox is created.
- **note** : this is only happens with first page.
- **video** :

4) crash when trying to working on a svg file that made from old non-multipage inkscape version.
- **Info.** : Inkscape crashes when trying to working with svg that made from old non-multipage inkscape version.
- **step to reproduce**
1) open svg file that is created using non-multipage inkscape version.
2) now add new page.
3) now try to save that doc. press ctrl+s.
4) crash.
```
ERROR:/home/harshil/GraphicsDesign/inkscape/src/page-manager.cpp:52:void Inkscape::PageManager::addPage(SPPage*): assertion failed: (it != pages.end())
Bail out! ERROR:/home/harshil/GraphicsDesign/inkscape/src/page-manager.cpp:52:void Inkscape::PageManager::addPage(SPPage*): assertion failed: (it != pages.end())
```
- **note** : probably same as this issue https://gitlab.com/inkscape/inbox/-/issues/6017
5) Compile error at src/ui/toolbar/page-toolbar.cpp line 138 `unit_str == backup;`
issue