Constraint mode is now switchable on/off in the effect screen.

I did some changes to the effect screen to make some space : instead of displaying an "on/off" label, we now keep the button pressed for active effects. 

There is no "constraint menu" yet, we should allow to select different constraints there (C64, MO5, spectrum, ...) to activate their respective enforcer and plug the right thing to the associated checker.

This should eventually allow drawers to :
 * Load any single-layer picture
 * Select the mode they want to use (without enabling it)
 * Run the checker and get the result as a multilayer (with an error layer)
 * Then, once they feel enough issues are solved, switch to "enforcer" mode for further manual tweaking.

Now we need a checker for CPC Mode5... :)

git-svn-id: svn://pulkomandy.tk/GrafX2/branches/cpcmode5@1721 416bcca6-2ee7-4201-b75f-2eb2f807beb1

src/buttons.h

@@ -438,6 +438,9 @@ void Button_Tiling_mode(void);
 */
 void Button_Tiling_menu(void);
 
+void Button_Constraint_mode(void);
+void Button_Constraint_menu(void);
+
 /*!
     Callback for the command that turns off all drawaing effects.
 */