magnifier: Add method to enable/disable crosshairs

These do some some signals connection and monitor layout tracking
that we forget to disconnect when the magnifier is disconnected.
This causes warnings if enabling the magnifier, disabling it,
switching to another tty, and back. (Presumably also happens for
monitor hotplugs).

This signal should just stay connected as long as it's needed,
so add a setEnabled() method on the CrossHairs actor that we
can hook on the magnifier.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1902>

js/ui/magnifier.js

@@ -172,6 +172,9 @@ var Magnifier = class Magnifier {
             this.stopTrackingMouse();
         }
 
+        if (this._crossHairs)
+            this._crossHairs.setEnabled(activate);
+
         // Make sure system mouse pointer is shown when all zoom regions are
         // invisible.
         if (!activate)
@@ -1677,9 +1680,7 @@ class Crosshairs extends Clutter.Actor {
         this._clipSize = [0, 0];
         this._clones = [];
         this.reCenter();
-
+        this._monitorsChangedId = 0;
-        Main.layoutManager.connect('monitors-changed',
-                                   this._monitorsChanged.bind(this));
     }
 
     _monitorsChanged() {
@@ -1687,6 +1688,16 @@ class Crosshairs extends Clutter.Actor {
         this.reCenter();
     }
 
+    setEnabled(enabled) {
+        if (enabled && this._monitorsChangedId === 0) {
+            this._monitorsChangedId = Main.layoutManager.connect(
+                'monitors-changed', this._monitorsChanged.bind(this));
+        } else if (!enabled && this._monitorsChangedId !== 0) {
+            Main.layoutManager.disconnect(this._monitorsChangedId);
+            this._monitorsChangedId = 0;
+        }
+    }
+
     /**
     * addToZoomRegion
     * Either add the crosshairs actor to the given ZoomRegion, or, if it is