Draft: unlocked_database: rework PopoverMenu

Menu looks very similar to what it was before, the only difference is that the buttons don't have tooltips, which were broken anyways according to #184 (closed).

So the cons are:

• No tooltips (maybe better than borked tooltips)
• Buttons are not marked as sensitive anymore, but as far as I am able to notice, that was not doing anything either

Since there is not real benefit from this other than cleaning the code, it might be added later, prior to a migration to gtk4.