Toolbar: Figure out why _addBackButton() is called twice when the toolbar has fixed positioning

[gabrielschulhof]

I came across this issue while fixing #6554. The quick solution was to check if I had already instantiated the back button, but we need to check why the function _addBackButton() is getting called twice during toolbar instantiation.

It looks like _addBackButton() is getting called from _setOptions() both times, so we need to figure out why _setOptions() runs twice during instantiation. It should not run at all.

[rmcdaniel]

I'm working on this now.

[rmcdaniel]

When I comment out this line, the issue disappears...

https://github.com/jquery/jquery-mobile/blob/master/js/widgets/fixedToolbar.js#L39

Obviously, this isn't a fix since now _makeFixed() is never called but it's getting us closer. Also, I see that _create() calls _makeFixed() and _makeFixed() calls _setOptions(), but _setOptions() calls _makeFixed() again. That doesn't seem right.