With 2.0.2 out the door and hopefully stable enough to keep people busy a couple of days, I'd like to start looking at metadata a bit.

NetCDF is quite cool in that respect, as it supports a lot of stuff:
- strings
- scalars
- vectors
- matrices
- per-primitive strings (aka attributes)

I'd like to reach the point where the sample file at the following location:
http://www.unidata.ucar.edu/software/netcdf/examples/GOTEX.C130_N130AR.LRT.RF06.PNI.nc
can be loaded "completely".

This commit adds support for data-source-level strings (global attributes), but there is a problem: I can't easily check them, as
the View->Strings dialog is too primitive and gathers only kst application-level strings, i.e. the strings defined in the
application itself.

We need to do something like for scalars, maybe even generalizing the ScalarTreeItem from scalarmodel to handle strings as well as scalars. I'm not a template guru, so help with that would be appreciated. I wouldn't want to start duplicating the code from the scalar case!