Open a menu upwards instead of downwards

[jsguay-haven]

This may be similar to #1176 but only in a general sense that if I use the egui::TopBottomPanel::bottom() and put a menu button in it, the menu still opens downwards so it's outside the screen.

Is your feature request related to a problem? Please describe.

I'd like to be able to have menu buttons on the egui::TopBottomPanel::bottom() panel, and have the menus open upwards.

Describe the solution you'd like

Ideally I think if a popup is going to open outside the screen it should be automatically repositioned to be inside the screen, as mentioned in the above issue.

Describe alternatives you've considered

I tried to write a version of popup_below_widget (called popup_above_widget) but I don't know how to get the size of the menu that would be created by add_contents(ui) in order to offset the popup (in .fixed_pos(...)). This is down to my lack of knowledge, so if this is something that's easy to answer, it may be sufficient to do what I want.

Additional context

Nothing to add really.

Thanks in advance!