The method is supposed to return the AbstractClient's size for a given
client size. That is the size including the window decoration.
The default implementation returned the passed in client size without
adjusting for the decoration. This resulted in ShellClient getting a
wrong size especially when AbstractClient::adjustedClientSize (which
calls sizeForClientSize) was called.
The result of the incorrect size was for example a shrinking of the
window when starting to resize a window.