Minibar improvement

This MR improves the Page Number toolbar item:

• The first commit disables visibility of the redundant label on creation, so that when Okular opens with no document, there is no redundant gap in the disabled minibar.
  • This is a fairly opinionated change, and can easily be dropped.

Before	After

• The second commit adds a tooltip to disambiguate between the page numbers visible. The editable field shows the page number on the page preferentially, or the actual page number, and the label if different shows the actual page number. The tooltip is only shown when both the editable field and the label are shown (which is when there is a need to disambiguate).
  • This was prompted by feedback from a user on confusion.