Buttons offset should automatically adjust according to observed scroll view's scroll insets

[romanr]

When it's added to TabBar, the scrollview extends under tab bar but it's scrollbars inset is within the viewport above TabBar.

Scrollbar does not extend under the TabBar. Scrollable area inset is known, PlusButton is expected to appear in that inset area.

Yes, we could fix it by setting:

self.plusButtonsView.offset = CGPointMake(0.f, -50.f);

But this is merely a workaround and may be broken in the future when different device and tabBar sizes introduced.

Expected: PlusButtons adjust inset according to observed scroll views scroll insets: