request: uiPopover

[andlabs]

migrated from andlabs/ui#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.

[andlabs]

May have to wait longer; the next GTK+ version we can step up to is 3.10, which does not have GtkPopover yet. (I could try faking it myself, but there's a quirk of the real one that would probably annoy people when I switch to the real one, and which does annoy the GTK+ devs: a GtkPopover cannot escape its parent GtkWindow, while my fake one would.)

[andlabs]

This is most likely going to be in a libui-future.