Merge Window::client{Minimized,Unminimized} with Window::minimizedChanged

This merges Window::clientMinimized() and Window::clientUnminimized()
signals with the Window::minimizedChanged() signal to simplify some
code.

The avoid animation flag has been dropped because its main usage is to
avoid playing animation during window initialization. But it's not possible
to trigger minimize animation at that moment. API-wise it's better to avoid
having such flags too and rely on surface role to decide whether to play a
given animation.