Hide windows instead of closing them when clicking on close

This implements event handlers to prevent the windows from being closed and hiding them instead. There is a difference between closing and hiding a window. When you close a window and open a project where this window is visible, it is empty, or in case of the FXMixerView, only displays the title bar. You can reproduce this by opening lmms, closing all windows and opening any project. All windows will be blank.