request: uiPopover

migrated from https://github.com/andlabs/ui/issues/36

There's some code for this in old ui already. It's not very elegant, and it's not themed, and it's still based on XP.

If I want this on GTK+ we either need to bump the version up or reimplement it ourselves. We can't use the official GtkPopover code because of LGPL.

There's code in various places, especially in the Windows backend, that assumes that a toplevel is a window. That will need to be fixed.