* Optimize StandardTableView in Fluent and Material styles
By using a ListView instead of a ScrollView + VerticalLayout, we trigger
the optimization where only visible children are being instantiated.
For the Fluent style, this was making the rows more compact, so I've
added additional padding to the TableViewCell to compensate. I don't
know where the padding used to come from.
I didn't touch Cupertino style since it's currently work-in-progress.
This optimization can be done there as well.
For the Native style, this optimization isn't as straight-forward
because it uses a NativeScrollView + Flickable directly rather than a
the ScrollView.
* Optimize StandardTableView for Native style
Using the ListView makes it possible to only instantiate the visible
items, which makes the StandardTableView a lot faster for larger models
(already very noticeably in the gallery example).
When using a ListView, StandardTableView lost access to the
NativeScrollView.native-padding-top/left properties, which are now
exposed through the ScrollView item.
It could also no longer position the Flickable below the header (or in
doing so, would take the scrollbar along with it), so a property was
added to ScrollView for that purpose as well.
* Introduced InternalScrollView
This avoids exposing additional properties on the ScrollView for just
the native style.
* Fix missing default values for Native style ScrollView
Added minimum size, preferred size and stretch factors (they were not
present before introducing InternalScrollView either, but are added now
for consistency with the other styles).
