plasmaquick: Handle enter/leave events in ResizeHandler

When the pointer leaves an applet window, the ResizeHandler will not unset the cursor shape. Futhermore, the cursor shape set by the resize edge handler will persist across QWindow::setVisble() calls.

In addition to that, QWindow::setVisible() will reapply the cursor shape when the window is shown. It seems like Qt ignores whether the pointer has entered the window. It is going to change the cursor shape regardless of the cursor position when QWindow::setVisible(true) is called.

This can result in weird bugs where the cursor changes its shape to Qt::SizeHorCursor or Qt::SizeVerCursor when clicking the system tray icon.