LibGUI: Add GUI_HOVER_DEBUG runtime debugging flag (environment)

You can now see the outline of GUI widgets when hovering them.
For example:

    $ export GUI_HOVER_DEBUG=1
    $ FileManager

Then move the mouse around in the file manager. :^)

Userland/Libraries/LibGUI/Widget.cpp

@@ -356,6 +356,11 @@ void Widget::handle_paint_event(PaintEvent& event)
         painter.draw_rect(rect(), Color::Cyan);
     }
 
+    if (app && app->hover_debugging_enabled() && this == window()->hovered_widget()) {
+        Painter painter(*this);
+        painter.draw_rect(rect(), Color::Red);
+    }
+
     if (is_being_inspected()) {
         Painter painter(*this);
         painter.draw_rect(rect(), Color::Magenta);