Add tabs adapter for TabLayout

TabLayout creates the tabs in an opaque, non-extensible way and there is no way to provide a custom view at creation time, you have to hack around it with an observer set on the content source (e.g., a ViewPager adapter) and then use setCustomView() on each tab, after they're created.

Besides, the way tabs' views are created prevents libraries such as Calligraphy from correctly applying a typeface to the default tab views, because the TextAppearance is applied only after inflating the view, and there's no way to access the tabs pool nor any hook into the creation of new tabs.

Suggested changes

Have a mechanism to create and bind the tabs' views, similar to list adapters, that have a createTabView() and a bindTabView() methods. The default implementation would simply retain the current behaviour; users could extend the default implementation to tweak the tabs' appearance (e.g., CalligraphyUtils.applyFontToTextView(tabTextView, typeface);) or to inflate their custom tab views without the waste of always inflating the default layout anyways.

Inizia dal percorso di creazione delle schede descritto nell’issue, inclusa la sua interazione con un adapter di ViewPager. Definisci come createTabView() e bindTabView() conserverebbero il comportamento predefinito consentendo al contempo viste personalizzate e l’applicazione del typeface; il lavoro è completo quando i chiamanti possono personalizzare la creazione senza osservare la sorgente del contenuto né inflatare un layout predefinito inutilizzato.