vcl: FontCharMap to use intrusive_ptr ImplFontCharMap

ImplFontCharMap was using it's own reference counting mechanism,
however we can use intrusive_ptr more effectively.

Added a unit test around FontCharMap.

Change-Id: Ifab6ce002fd1df8feb7e017dea3012ff9ea7f18a
Reviewed-on: https://gerrit.libreoffice.org/11804
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>