Custom groups

As discussed in the forum thread, this adds the ability to create and manage custom groups and the dwarves who love them.

Right click a dwarf name to add a new group, delete an existing one, or add/remove from a group. Works with multiple selections.

Group names are global, but dwarf/group assignments are per-fortress.

(Also has the fix for OS X and setuid detection; I can split that out if you want.)

I've never done any Qt before, so please don't be shy about pointing out where I've been dumb.
Consider passing the <code>QSettings</code> into the <code>CustomGroup</code> object to move the loading into the object itself. See <code>labor.h</code> for a simple example.