Minor refactoring in VirtualFlow.
Add more methods to VirtualFlow.
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.
Reduce unnecessary paragraph updates.
Add VirtualFlow.hit(double offset) method.
Add VirtualFlow.show(cell, region) method.
Print errors encountered during event processing.
VirtualFlow: Fix off-by-one error.
Fix Up/Down arrow navigation when the caret is not in the viewport.
Remove commented out and unused code.
VirtualFlow: Use Bounds instead of Rectangle as cell region specification.