We want to start to decouple Window from OutputDevice. This is
because a Window should *use* an OutputDevice, a Window is *not*
an OutputDevice.

In order to start to decouple the two classes, the strategy will be:

1. This patch adds in a new private member pOutputDevice as well
   as an accessor. When the Window is initialized, pOutputDevice
   is set to a downcasted instance of this (because we are still
   inheriting Window from OutputDevice).

2. We will start to go through all the functions in Window that rely
   on OutputDevice and use the getter function to use the OutputDevice
   private member.

3. Any subclasses of Window will then need to have the same procedure
   done on them.

4. Once this is done, we need to work out the best way of initializing
   mpOutputDevice, then we can remove OutputDevice as the parent
   class of Window (and all Window subclasses).

Change-Id: I39886c77070befb1542c8f22346111cde1c360b1
Reviewed-on: https://gerrit.libreoffice.org/7786Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>