VirtualFlow: Introduce Cell interface.
Parameterize VirtualFlow by cell type.
VirtualFlow: Add CellWrapper class.
VirtualFlow: Expose some public methods.
VirtualFlow: Help the compiler with type inference
VirtualFlow: Fix cell positioning.

Correctly position cells even when they exceed their layout bounds.
VirtualFlow: Add style classes.
VirtualFlow: fix cell indices on item list updates
VirtualFlow: add dispose() method.
Use VirtualFlow to implement StyledTextArea skin.

Replaces custom hacks to ListView.