Chart context menu was misplaced since the context menu expects a
position related to the document window not to the chart window.
Moreover map mode is not enabled for chart window, that leads to use
the LogicToLogic routine which unfortunately provides a wrong pixel to
twips conversion. In fact we gets a 20 twips per pixel conversion
since LogicToLogic uses a fixed 72 dpi value, instead of a correctly
computed output device real dpi or at least the most commonly used 96
dpi value.

Change-Id: Idb4220d03fddd4b6d643c1c0fb52754bafed7e36
Reviewed-on: https://gerrit.libreoffice.org/53118Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/54649
Reviewed-on: https://gerrit.libreoffice.org/54655Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Marco Cecchetti <mrcekets@gmail.com>