ComboBox: List-mode popup must open on CB’s screen

ComboBox: List-mode popup must open on CB’s screen

The screen for the list-mode popup_window was only being set in
set_popup_widget(), i.e. when changing modes, so if the ComboBox was
moved to a different screen later, the popup would appear on the
original one, which is wrong.

Worse, this (somehow) broke opening some combos in the Inspector.

Fix this by moving the call to set_screen() to popup_for_device(), so
the popup_window is put on the correct screen each time around.

https://bugzilla.gnome.org/show_bug.cgi?id=468868
https://bugzilla.gnome.org/show_bug.cgi?id=786771