Moved all the casts inside ::createFastChildContext and
used reference to it as an argument in the constructors. This
avoids spreading the cast statements all over the place.

Also removed some of the empty createFastChildContext(), they are
unnecessary as the parent class already has it.

Change-Id: I344ede732a53878a7e265c0178b07d73b5398237
Reviewed-on: https://gerrit.libreoffice.org/41178
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>