entry completion: Use a popover

entry completion: Use a popover

This lets us remove a use of GTK_WINDOW_POPUP,
which should eventually be going away.

We need to disable treeview search, since it
creates a toplevel that will disrupt our grabbing
popup, causing it to be dismissed.

We don't need to grab ourselves, since the popover
code does it for us. We don't need to reposition our
window, since the popover takes care of that too.