1) I would like to integrate new icons on the menu bar at the top ... What lines and files to modify?

Only way is to change all icons of corresponding item.
You can do it both ways :
- directly replace icons in /view/css/images
- add you own icons in /view/css/images and change css description in /view/css/projectorIcon.css

How to change the order of icons in the top menu ?

Change the order in the table "menu". Take care that it will also change order in the left menu, and order must be coherent with top menu of each item.

I would like to place in the top menu icon to go directly to the list of connected users

You must change restriction clause in /view/menuBar.php, line 39

    	if ($menu->id==36) {$suspend=true;}

to allow menu code you'd like. Actually, all menus with order > order of menu 36 won't be displayed.

1) About Question 1: More specifically, my problem is to add new icons for calling new functions ....

What files and lines to change?

Cordially

1) About Question 1: More specifically, my problem is to add new icons for calling new functions ....
What files and lines to change?

- Add corresponding lines it table menu.
- Add corresponding habilitations (throught sql or through access right screen)
- Add icons description in /view/css/projectorIcon.css (copy an existing for help)
_ then maybe add some code in /view/menuBar.php (you'll have tofind where as it depneds on your need...)

"...to allow menu code you'd like. Actually, all menus with order > order of menu 36 won't be displayed. "

There is a small problem : when I put the value $menu == 37 ($menu->id==37), twenty additional icons appear. I understand that incrementing the value of 1 (36 => 37), had the effect of displaying an additional icon on the top menu bar, and so on ...

How to display some specific icon precisely ?

Read the code !
Replacing

    foreach ($menuList as $menu) { 
    	if ($menu->id==36) {$suspend=true;}
    	if (! $suspend and securityCheckDisplayMenu($menu->id,$menu) ) {
    		drawMenu($menu);
    		$lastType=$menu->type;
    	}
    }

with

    foreach ($menuList as $menu) { 
    	if ($menu->id==36) {$suspend=true;}
    	if (! $suspend and securityCheckDisplayMenu($menu->id,$menu) ) {
    		drawMenu($menu);
    		$lastType=$menu->type;
    	}
    }

is not what you need.
You should rather set somethong like

    foreach ($menuList as $menu) { 
    	if ($menu->id==36) {$suspend=true;}
    	if ( (! $suspend or $menu->id==xxx) and securityCheckDisplayMenu($menu->id,$menu) ) {
    		drawMenu($menu);
    		$lastType=$menu->type;
    	}
    }

where xxx is the id of the menu you want to show.