I found this part in GoogleMapIntegration.php, which seems to be the right place to start with building a how-to how to add menu buttons:
/**
* integrate_menu_buttons
*
* - Menu Button hook, called from subs.php
* - used to add top menu buttons
*
* @param mixed[] $buttons
* @param int $menu_count
*/
function imb_googlemap(&$buttons, &$menu_count)
{
global $txt, $scripturl, $modSettings;
loadlanguage('GoogleMap');
// Where do we want to place our button (new menu layout, this needs to be redone)
$insert_after = empty($modSettings['googleMap_ButtonLocation']) ? 'memberlist' : $modSettings['googleMap_ButtonLocation'];
$insert_after = 'memberlist';
// Define the new menu item(s), this will call for GoogleMap.controller
$new_menu = array(
'GoogleMap' => array(
'title' => $txt['googleMap'],
'href' => $scripturl . '?action=GoogleMap',
'show' => !empty($modSettings['googleMap_Enable']) && allowedTo('googleMap_view'),
)
);
$buttons['home']['sub_buttons'] = elk_array_insert($buttons['home']['sub_buttons'], $insert_after, $new_menu, 'after');
}
Seems there only must be changes made if the location of the button is different. I can't write code so I don't want to write an example how-to for this, but maybe someone of you can?