- added 'Delete' button in visibility browser so that we can easily remove
  physicals and partitions from a model

- slightly changed the behaviour of "Save all (discard physicals)" in the msh
  format: now we really discard the physicals, and set the physical-region
  tag to 0 in the msh file. This is the most natural thing to do: this
  way when on reads a msh file, we get the correct information back.