LibGUI: Update TextEditor after triple-click to select (#4897)

Previously, it was "relying" on the cursor blink timer to update
the visual selection.

This caused some delay after the whole line was selected by
triple-clicking specifically the last word in a line.

Now, the widget is updated after triple-clicking.

Libraries/LibGUI/TextEditor.cpp

@@ -286,6 +286,8 @@ void TextEditor::mousedown_event(MouseEvent& event)
 
         m_selection.set(start, end);
         set_cursor(end);
+        update();
+        did_update_selection();
         return;
     }