gridvis-desktop.preinst: adding installer helper

The gridvis-desktop is creating some caching informations while a first
startup. In case of a package update the new gridvis-desktop package
would use these old cached informations, this is a unwanted behaviour.
In case of a package update we have to move out this cached informations
of the way. Doing this by using preinst helper mechanism.
d/g-d.postrm: change purging depending on gridvis-server

The package 'gridvis-server also uses the folder
/var/lib/GridVisProjects. So not purging the folder if the package
'gridvis-server' seems to be installed.