When the field in question is read from inside a constructor
initializer.

In the process, create some needed infrastructure in the plugin classes.

Change-Id: I2f440efa6912801a236727c9fe3180404616958c
Reviewed-on: https://gerrit.libreoffice.org/38960Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>