Instead of learning scripting and all that that the others suggested there is a much easier way.1: Make spring windowed
Go to your "Spring Settings". Make sure that "Expert Mode" is chosen. Then go to "Render quality/Video mode" and uncheck "Run full-screen".
Now the game will run always in a window.2: Make a shortcut to un/fullscreen
Go to System>Preferences>Keyboard Shortcuts. There under "Window Management" should be an option "Toggle fullscreen mode" and next to that there is a key shortcut that you can change to your own preferences. I have set ALT+F to be my shortcut.
So next time you run spring just press the shortcut(ALT+F in my case) and it should work. :)
p.s. now you can run fullscreen ANY program by just pressing the key shortcut
edit: by the way if that doesn't work, while you are in the "Keyboard Shortcuts" click on "Add" and as Name give something like "Toggle to Fullscreen". As command add this:
wmctrl -r :ACTIVE: -b toggle,fullscreen
That needs wmctrl that you can download from Synaptic. I just checked it and that's what I use.