Fix initial selected tab not visible

[joshdifabio]

If the tabs are scrollable and the initial selected tab is not visible (i.e. it is off to the right), it should be scrolled to automatically. At present, this is not the case, because _scrollToSelectedIfNeeded() is not called when there is no 'old' tab.

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[joshdifabio]

I've now signed the CLA.

This PR should fix #175.

[googlebot]

CLAs look good, thanks!

[joshdifabio]

@bicknellr Any chance you could take a look at this please? :)

[paladox]

Need to fix merge conflicts.