Improved plotting of NXlog directories

Added the start time to the plot title
Updating version number to v0.4.6

Ready for next release
Reconfigured PlotDialog and SignalDialog

Now, it is possible to select a group in the tree, and then select a
signal within the group. Also improved error reporting when there are
no axes.
Reconfigured use of the 'entries' property

This returns the group._entries dictionary, but this will not parse
full paths in the way that __getitem__ does. So I now only use _entries
when I know that a full path is inappropriate. Also, renames of NXlinks
raises an error.