gitk: Add a window to list branches, tags and other references

gitk: Add a window to list branches, tags and other references

This adds an entry to the File menu labelled "List references" which
pops up a window showing a sorted list of branches, tags, and other
references, with a little icon beside each to indicate what sort it
is.  The list only shows refs that point to a commit that is included
in the graph, and if you click on a ref, the corresponding commit
is selected in the main window.  The list of refs gets updated
dynamically.

Signed-off-by: Paul Mackerras <paulus@samba.org>