[Ide] Fixing disabled OSX ContextMenus in Modal dialogs(Bug 20683)

Only thing I'm not sure is if
```csharp
if (commandSource != CommandSource.ContextMenu) {
   item.Enabled = !IsGloballyDisabled && info.Enabled;
}
```
should be
```csharp
if (commandSource == CommandSource.MainMenu) {
   item.Enabled = !IsGloballyDisabled && info.Enabled;
}
```
or something more complex...