refactor(ripple): adds support for non-fixed tabs

Closes #825.
Closes #460.
Just to note, we are talking about the demo here, and not the core inkBar styles.  When I add transitions between colors, I plan to optimize the demo so that it fully showcases that, which will definitely require some colors changes.  (For now, I made all content backgrounds the same color because the hard color change did not look right.)

Also, I think that's a good idea - we should definitely include `$tabs-inkbar-color` in `tabs-theme.scss`.