Document VCL Application class

Documented the VCL Application class in Doxygen format. Also
reindented some of the code, and moved ValidateSystemFont down
the list of class functions (better code organization).

Change-Id: Id421647a9a25e11be87b0f6c46d1ac0f4846f6a4
Reviewed-on: https://gerrit.libreoffice.org/7515
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>