Bug 407884

SUMMARY
Just discovered that the context menu in the Contents side panel offers the two actions “Previous Bookmark” and “Next Bookmark”.
They are put in a section named "Page N", where N is the *number* of the page to which the current item in the Contents tree view points.
Nevertheless, these two actions use the current viewport for determining the next/previous bookmark, not the page N like the section title suggests.

STEPS TO REPRODUCE
1. Open a document with many pages and TOC
2. Create some bookmarks roughly at 1/3 and 2/3 of the document
3. Go to the middle of the document
4. Open the Contents panel
5. Open the context menu on the first item.
6. The section “Page 1” contains the action “Next Bookmark”

OBSERVED RESULT
Okular::Part::m_nextBookmark is triggered and brings me to the bookmark at 2/3 of the document.

EXPECTED RESULT
The next bookmark from page 1 is at 1/3 of the document, so it brings me there.
I expect the action to respect the page number in the section title, because the other actions in the section (“Add Bookmark”) also do.

SOFTWARE/OS VERSIONS
KDE Neon 5.15.5
KDE Plasma 5.58.0
Qt 5.12.0
Okular 1.7.1

ADDITIONAL INFORMATION
The context menu ist built in Okular::Part::slotShowTOCMenu(), which calls Okular::Part::showMenu().
This adds the actions “Add Bookmark” / “Remove Bookmark” and “Fit Width” to the context menu.
These actions are custom made for page N.
However, it also adds m_prevBookmark and m_nextBookmark, which don’t know anything about page N. When triggered, they just fetch the current viewport.

ADDITONAL ADDITIONAL INFORMATION
In the summary, I wrote: "Page N", where N is the *number* of the page.
However, the Contents tree view items show the *name* of the page. This may be confusing, because the user might see two different numbers in the tree view item and the context menu.