Update WindowItem's visibility

With this, the WindowItem will know whether it's actually visible. As
the result, if a native wayland window has been minimized, kwin won't
try to schedule a new frame if just a frame callback has been committed.

EffectWindow::enablePainting() and EffectWindow::disablePainting() act
as a stone in the shoe. They have the final say whether the given window
is visible and they are invoked too late in the rendering process.
WindowItem needs to know whether the window is visible in advance,
before compositing starts.

This change replaces EffectWindow::enablePainting() and
EffectWindow::disablePainting() with EffectWindow::refVisible() and
EffectWindow::unrefVisible(). If an effect calls the refVisible()
function, the window will be kept visible regardless of its state. It
should be called when a window is minimized or closed, etc. If an effect
doesn't want to paint a window, it should not call effects->paintWindow().

EffectWindow::refVisible() doesn't replace EffectWindow::refWindow() but
supplements it. refVisible() only ensures that a window will be kept
visible while refWindow() ensures that the window won't be destroyed
until the effect is done with it.