Bug 1978425: External -> Plugins page to show title / version

- Currently also displaying the old plugin string to help with title
diagnosis

- To change the title on a blocktype you need to add the
'get_plugin_display_name' function to the blocktype,

eg for watchlist you need to add

    public static function get_plugin_display_name() {
        return get_string('pluginname', 'blocktype.watchlist');
    }

Inside the PluginBlocktypeWatchlist class

Change-Id: I33ea544d6cfee22b2e7fecb7fa99a485aea6a8f2
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>