Respond to lost connection better in gcylc suite views.

View-clearing code was called repeatedly instead of just once when not connected to the suite. In the graph view this involves a graphviz subprocess, repeated calls to which could result in hundreds of error dialogs if the subprocess was failing for some reason.

Also tidies a bit of old code, splits the dot- and tree-view updater classes into separate files like the graph-view updater, and renames a couple of files more sensibly.

@benfitzpatrick - please review.  Note this is quite a small change - splitting the aforementioned file in half makes it look large (superficially).