Was failing to show, with this warning:

Gdk-WARNING **: Tried to map a popup with a non-top most parent

To make it work, needed to implement a way to pass as extra
"ParentWindow" property to the ui element. It's documented in the
XUIElementFactory idl, but was not used by the framework impl.

Change-Id: Ifea24fc333709478634f70230699963e952e9157
Reviewed-on: https://gerrit.libreoffice.org/42937Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>
Tested-by: Maxim Monastirsky <momonasmon@gmail.com>