Make a kit's file system friendly name configureable.

This makes it possible to use concise names for shadow-build
directories, for example {projectname}_5s for Qt 5 / stable instead
of the long, auto-generated names. Also, users can then manually
configure names for kits with non-latin names, which would otherwise
result in underscores and dashes with numbers.

Change-Id: If0eab746942246d1aba6a5dd04f59d3219be47b8
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>