Cover X11Window by tests

X11Window misses a lot of tests. For example, for its various placement heuristics in X11Window::manage() or even basic functionality such as window minimization or maximization.

• maximized horizontally and vertically, minimized, fullscreen, keep below, keep above
• virtual desktops: set/unset "on all desktops", etc
• ConfigureRequest
• NETMoveResize
• stack requests
• opacity
• skip pager/skip switcher/...
• color scheme
• modal dialogs
• ASN
• WM_TRANSIENT_FOR