Add label to indicate focusing ability

src/Tools/tim_tool/ui/tab/main-tab.slint

@@ -181,6 +181,13 @@ export component MainTab inherits Rectangle {
                                 }
                             }
                         }
+                        HorizontalLayout {
+                            padding: 4px;
+                            alignment: center;
+                            Text {
+                                text: "Right click to focus element on screen";
+                            }
+                        }
                         HorizontalLayout {
                             padding: 4px;
                             Button {