Fix Yakuake icon appearing in taskbar through Qt::Tool window flag

So far we've been setting _NET_WM_STATE_SKIP_TASKBAR ourselves, but
Qt messes with this, leading to race conditions where it still
appears in the taskbar. This sets the Qt::Tool window flag, which
will set it via Qt itself, and won't be clobbered. An old hack has
been removed, as it isn't needed anymore.

Also makes show animation match retract animation.

BUG: 360037