Add tooltip to horizontal-bar-tabs & :validate?

[PawelStroinski]

The new attributes:

• :tooltip-fn [horizontal-bar-tabs only] given an element of :tabs, returns its tooltip
• :tooltip-position [horizontal-bar-tabs only] relative to this anchor. One of :above-left, :above-center, :above-right, :below-left, :below-center, :below-right, :left-above, :left-center, :left-below, :right-above, :right-center, :right-below
• :validate? [horizontal-bar-tabs & vertical-bar-tabs only] validate :model against :tabs (true by default as this is the current behaviour)

The Tooltips demo has been added as well.

[superstructor]

Thanks @PawelStroinski 👍 😄

Tested locally via demos page and merged.