HexEditor: Add toolbar and context menus to the Annotations panel

The Annotations panel is the most obvious place to perform actions
related to annotations, so let's make that possible. :^)

The toolbar gets open/save/save-as actions for annotations, and one for
adding an annotation. The table itself gets a context menu for editing
or deleting the selected annotation.

Userland/Applications/HexEditor/HexEditorWidget.gml

@@ -60,6 +60,14 @@
                 preferred_height: "grow"
                 visible: false
 
+                @GUI::ToolbarContainer {
+                    name: "annotations_toolbar_container"
+
+                    @GUI::Toolbar {
+                        name: "annotations_toolbar"
+                    }
+                }
+
                 @GUI::TableView {
                     name: "annotations"
                 }