Legend improvements - further to #117

[morb_au]

Whilst the immediate issue in #117 has been addressed, I have come up with some further ideas that may be useful to implement in the post-0.8 timeframe.

1. Overview and Editing icons can be also shown beside the top-level layer name, to avoid having to enable "Show file groups".
2. There also seems to be a "projection not valid" icon floating around in the code, however this hasn't been addressed in r5594. Perhaps we need to do something for it too.
3. The column sizes have been hard-coded to arbitrary values. I have no problem if these get tweaked further.
4. Add an asterisk (*) to the layer name if it *has* been edited but not saved. This is similar to some text editors I have seen.