Is your feature request related to a problem? Please describe.
I use xournalpp without the sidebar and recently spent a long time trying to figure out how, or if it was possible, to reorder pages. Looking through the wiki and web site and I could not find any mention but some issues here said "right click on the page and select move up" but right clicking on a page's blank area did nothing for me except raise blood pressure.

Eventually I stumbled upon the sidebar.

Describe the solution you'd like
Selecting a page or object on a page highlights that page and it's border goes red to indicate it is the active page.
Menu commands such as "new page before" "new page after" work on this active page's index so...

I propose adding menu command "Move page up" and "Move page down" to the Journal menu.

Describe alternatives you've considered
Right clicking on a page brings up a context menu with Journal menu and such things as "Move up"

Additional context
Would be helpful to add something in the wiki or on the website guide, though I'm not sure which is used most or maintained. Anyway this issue is at least searchable for people to find.