ImplPopupFloatWin draws the window title and frame directly on the client
window, which is incompatible with .ui layouts, as they try to occupy the
whole available space. Instead just use a plain FloatingWindow whenever
possible, and change the border window code to mimic the behavior of
ImplPopupFloatWin, wrt removing part of the frame of toolbar popups w/o a
title.

Change-Id: If72ea89e31f913470faad7fcb8083f9b9787c9a8
Reviewed-on: https://gerrit.libreoffice.org/48094Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>
Tested-by: Maxim Monastirsky <momonasmon@gmail.com>
(cherry picked from commit 829b0564)
Reviewed-on: https://gerrit.libreoffice.org/48202Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>