Emit clientMaximizedStateAboutToChange when maximized mode is actually about to change
Window rules code can call maximize(requestedMaximizeMode()), in which case the Window will emit clientMaximizedStateAboutToChange but the maximize mode may not actually change.
This change moves the emission of of the clientMaximizedStateAboutToChange signal to Window::changeMaximize(). The reason for doing so is that window rules have the final decision what the maximize mode will be.