A save dialog should be offered when leaving TA (if something has been done)

[rgs]

This is on Gnome (and probably any other Desktop environment that is not Sugar).

Steps to reproduce the bug:

1) start TA
2) do something (i.e.: a simple program in which the turtle walks forward)
3) close TA

and no "save" dialog is offered.

[rgs]

Btw, do we have something like canvas_has_blocks()? Or start_has_blocks()? If we don't, how hard would it be to implement such method.

[rgs]

Here is an initial attempt. It handles the basic case:

1) start TA
2) do something
3) try to quit. if something was done, it'll ask if you want to save before quitting

Case not being handled(yet):
1) load an existing project
2) don't do anything
3) it'll still ask if you'd like to save (as if it was a new project)

[walter]

in v89