right, for true functionality you'll need to make all teh panes of glass seperate.
name each pane window1 or whatever. ie: all panes of glass (you'll need to make all the panes func_breakable seperately], don't group them all at once and make them a func_break)
Now make the frame, and group all the seperate brushes you used to make the frame and make them a func_breakable (so you have One func_breakable for the frame, and many func_breakables for teh glass panes.
Now, in the frame's target line, type: window1 (or whatever you named teh panes of glass.
What happens is that when you shoot the glass, onkly 1 pane will break at a time, if you shoot the frame, everything breaks.