Add Gnome Shell 46 compatibility

Replace removed Clutter methods:
  - add_actor() -> add_child()
  - remove_actor() -> remove_child()

System_Monitor@bghome.gmail.com/metadata.json

@@ -1,10 +1,10 @@
 {
-	"shell-version": ["45"],
+	"shell-version": ["45","46"],
 	"uuid": "[email protected]",
 	"name": "System Monitor",
 	"description": "Display resource usage.\n\nLinux distribution specific installation instructions can be found in the wiki at https://github.com/elvetemedve/gnome-shell-extension-system-monitor/wiki/Installation.\n\nPlease report bugs here: https://github.com/elvetemedve/gnome-shell-extension-system-monitor/issues",
 	"settings-schema": "org.gnome.shell.extensions.system-monitor",
 	"gettext-domain": "[email protected]",
 	"url": "https://github.com/elvetemedve/gnome-shell-extension-system-monitor",
-	"version": "34"
+	"version": "35"
 }

System_Monitor@bghome.gmail.com/view.js

@@ -38,7 +38,7 @@ class Menu extends PanelMenu.Button {
                 }
             }
         });
-        this.add_actor(this._layout);
+        this.add_child(this._layout);
 
         this._initIconsAndWidgets();
         this._addPositionSettingChangedHandler();

System_Monitor@bghome.gmail.com/widget.js

@@ -91,7 +91,7 @@ class BaseMenuItem extends PopupMenu.PopupBaseMenuItem {
         }
 
         if (position != -1) {
-            this.actor.remove_actor(this.icon);
+            this.actor.remove_child(this.icon);
             this.icon = icon;
             this.actor.insert_child_at_index(this.icon, position);
         }
@@ -141,13 +141,13 @@ class ResourceTitleItem extends St.Widget {
             style_class: 'resource-title'
         });
 
-        this.add_actor(icon);
+        this.add_child(icon);
 
         let leftLabel = new St.Label({text: text, style_class: 'resource-title-label', x_expand: true, y_expand: true, x_align: Clutter.ActorAlign.START, y_align: Clutter.ActorAlign.CENTER});
-        this.add_actor(leftLabel);
+        this.add_child(leftLabel);
 
         this.#rightLabel = new St.Label({text: summary_text, style_class: 'resource-title-label', x_expand: true, y_expand: true, x_align: Clutter.ActorAlign.END, y_align: Clutter.ActorAlign.CENTER});
-        this.add_actor(this.#rightLabel);
+        this.add_child(this.#rightLabel);
     }
 
     setSummaryText(text) {