Add group-all/ungroup-all buttons to the graph view tool bar, and ...

... disable grouping to root level, which is essentially useless.
Merge pull request #362 from hjoliver/gcylc-graph-view-grouping-changes

Add group-all/ungroup-all buttons to the graph view tool bar, and ...
Small hot fix: previous change broke right-click menu...

on gcylc dot and text view.