Fix "Viewer.buttons is deprecated" warning issue

[joedf]

Removes the erroneous Viewer.buttons is deprecated; Please use Viewer.buttonGroup warning. See explanation in #2193 ...

[iangilman]

Thank you for taking this on!

I don't think this is enough. What we really want is to be copying the getters and setters into the new object, but the current state of this patch will copy undefined for anything that has a getter (since the property descriptor for something with a getter has no value). That does indeed fix the warning, but it'll lead to unintended behaviors.

I've messed around, and I think this works:

                    var descriptor = Object.getOwnPropertyDescriptor(options, name);
                    if (descriptor.get || descriptor.set) {
                        Object.defineProperty(target, name, descriptor);
                        continue;
                    }

                    copy = descriptor.value;

                    // Prevent never-ending loop
                    if ( target === copy ) {
                        continue;
                    }

                    // Recurse if we're merging plain objects or arrays
                    if ( deep && copy && ( OpenSeadragon.isPlainObject( copy ) || ( copyIsArray = OpenSeadragon.isArray( copy ) ) ) ) {
                        src = target[ name ];

Here's what it looks like as a diff in the context of the code:

What I found is that both target[name] and options[name] can trigger the warning. To take care of the options version, we are getting the descriptor and then setting the property on the target if needed. To take care of the target version, I'm simply moving it into the only clause where it's being used, and from there it doesn't seem to trigger any of our getters.

@joedf What do you think?

[joedf]

@iangilman I've put your suggested changes in. Seems to work great!

[iangilman]

Awesome... Thank you for verifying the fix and getting it in!