Update MantisGraph to print submenu

Previously, the plugin's submenu was handled as an event hook, adding
buttons to the Summary page via EVENT_SUBMENU_SUMMARY.

Since the graphs are now grouped under their own tab, it does not make
sense to rely on that hook anymore, so the plugin's summary_submenu()
method was renamed print_submenu() and the code updated to use the
HTML API's eponymous function.

The plugin also defines and signals a new EVENT_MANTISGRAPH_SUBMENU
event, allowing 3rd-party plugins to add additional graphs.