Add default vertical scrollbar to TableView

[AndrewSazonov]

This PR adds the existing EaElements.ScrollBar as the default vertical scrollbar for TableView with AsNeeded policy and interactive behavior enabled.

[seventil]

You can add topInset and topPadding to the ScrollBar if you want to ofset height for the header.
I guess with AsNeeded policy user only sees a narrow indicator-like bar anyway, and they can interact with it if they want, so it's totally OK to define interactive:true.

Here's an example from listview just for reference:

property real _headerHeight: headerItem ? headerItem.height : 0
ScrollBar.vertical: EaElements.ScrollBar {
        policy: listView.scrollBarPolicy
        interactive: listView.scrollBarInteractive
        topInset: listView._headerHeight
        topPadding: listView._headerHeight
    }

[AndrewSazonov]

Right, I forgot about this, thanks for pointing it out.

I would avoid adding an extra _headerHeight QML property unless it is really needed, since every QML property also adds the associated signal/slot machinery. In this case the value is only used by the scrollbar, so keeping it inline is simpler.

Also, I used listView.headerItem instead of just headerItem to make it clearer that the height comes from the parent ListView.

ScrollBar.vertical: EaElements.ScrollBar {
    topInset: listView.headerItem ? listView.headerItem.height : 0
    topPadding: topInset

    interactive: true
    policy: ScrollBar.AsNeeded
}

[seventil]

LGTM