Add project.rootdir that always points to toplevel
Add project.rootdir that always points to toplevel

This is a bit of a burden to maintain when projects are added or moved,
but makes it easier to point to files off the root path. Usually doing
so is a bad idea anyway, but in some cases it is nice to have access to
shared configuration, such as the files in the config folder.

This used to be revision r5471.

imagej/imagej-launcher,imagej/imagej-launcher,imagej/imagej-launcher