Document Application class

Documented more of the VCL Application class's public functions
via Doxygen markup.

Also, change from global groups to member groups.

Change-Id: I5e80e707874db99c60e899d078f41c7817e2b6c3
Reviewed-on: https://gerrit.libreoffice.org/7664
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>