General plugins

Ok so first and foremost I am new to Moodle Development and I am working at a great new job building on to an existing Moodle application. Currently I have to develop what should be a simple plugin to add a dropdown inside of all of the pages after a student or instructor logs in. 

I have seen multiple tutorials on plugins thorugh your docs but have not seen any plugins that span multiple pages.  So here are the details on what I am trying to make.

I want to build a toolbox that sits in the header and spans multiple pages.

I really would like to keep from touching the theme file too much as there are multiple themes and this can be a problem.

I definitely cannot touch any core moodle files as I was told not to LOL.

and here are some other specification:

currently I have some semblance of a plugin built.

I have the plugin outputting mustache files that will eventually query the Database for user selected items.

All in all I am very very new to moodle. If there is a tutorial out there for this that would be great. All of the tutorials I have encountered thus far have been to build a plugin but then that becomes its own page so to speak. As I said above I need this to span multiple pages (like a widget). 

If there are no tutorials out there I would settle on how I can get the templates  to just display without touching any core files. I was able to get it to display earlier by taking the following code and placing it in the outputrenderers.php but that idea is a no go per my boss LOL:

public function toolbar_menu(){
    global $PAGE;
    global $OUTPUT;
    $renderable = new \local_toolboxtest\output\menu_wrapper();
    $output = $PAGE->get_renderer('local_toolboxtest', 'menu_wrapper');
    return $output->render($renderable);
}