Draft: Default page context menu fix

Fix for issue #5412

Thanks for the heads-up, @doctormo ! 🙌

There are different types of SVG files.
SVG files with no pages. NO PAGES. This means the manager doesn't have any SPPage objects and has a thin interface to direct any requests for pages to the CANVAS where possible.
SVG files with with pages. ONE OR MORE. This means the page manager can give a list of pages. The canvas is usually kept in sync with the FIRST PAGE if it's already in sync.
This is deliberate design so Inkscape can support and understand zero-page SVGs. And I think a lot of people get confused because they see that white square that looks like a page and think quite naturally "that is a page". But it's not a page, it's a canvas pretending to be a page.

Solution: create a page out of the canvas (in case of zero-page SVGs)

Before fix (context menu can't pop up on default page):

Before_bug_fix

After bug fix:

After_bug_fix