detect static variables that can be made const.

Thanks to mike kaganski for suggesting this.

Here I introduce a new plugin feature - using markers
in nearby comments to disable the plugin for specific
vars.

Some of this stuff was old debugging code. I removed the stuff
that was older than 5 years.

Change-Id: I6ec7742a7fdadf28fd128b592fcdf6da8257585c
Reviewed-on: https://gerrit.libreoffice.org/68807
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>