Put all the UI content in UI XML file.

Unfortunately, lots of boilerplate code because
G_DECLARE_* macros are available only since glib 2.44

Change-Id: Idc74ba8565d482c28abd00b6f6f75646ab3d40b9
Reviewed-on: https://gerrit.libreoffice.org/39913Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: pranavk <pranavk@collabora.co.uk>