1. Consolidate linking enable/disable logic b/w Edit Toolbar and Menu

2. Save user's preference for enabling linking

3. fix a bug by which changes to global prefs like SWPlaythrough were not
visually reflected in other open projects.

src/Menus.h

@@ -37,6 +37,8 @@ void AddEffectsToMenu(CommandManager* c, const EffectSet& effects);
 void CreateRecentFilesMenu(CommandManager *c);
 void ModifyUndoMenus();
 void ModifyToolbarMenus();
+// Calls ModifyToolbarMenus() on all projects
+void ModifyAllProjectToolbarMenus();
 
 int GetFocusedFrame();
 wxUint32 GetUpdateFlags();