keyboard handling change

[Salvador Cuñat]

Hi.
I can't compile this patch,  I get:

gtk-gui.c: In function ‘on_key_press’:
gtk-gui.c:1154: error: ‘GDK_KEY_Up’ undeclared (first use in this function)
gtk-gui.c:1154: error: (Each undeclared identifier is reported only once
gtk-gui.c:1154: error: for each function it appears in.)
gtk-gui.c:1157: error: ‘GDK_KEY_Down’ undeclared (first use in this function)
gtk-gui.c:1160: error: ‘GDK_KEY_Left’ undeclared (first use in this function)
gtk-gui.c:1163: error: ‘GDK_KEY_Right’ undeclared (first use in this function)
make: *** [gtk-gui.o] Error 1

Obviously it's  because I'm using GTK2.

I think in the next release of subsurface will be necessary to include some 
minimum requirements.

Regards.

Salva.