WIP: add VirtualizingGridPanel

[ahopper]

• What does the pull request do?
  Adds a VirtualizingGridPanel that displays as many fixed sized item as fit into the container and allows proper scrolling on a line by line basis
• What is the current behavior?
  no such control, attempts to create virtualizingpanels without core changes can only scroll an item at a time rather than a line at a time.
• How was the solution implemented (if it's not obvious)?

IVirtualizingPanel was extended to include a ScrollQuantum which is used by ItemVirtualizerSimple to scroll properly.
Whilst this works perfectly for the VirtualizingGridPanel and the existing VirtualizingStackPanel I'm not sure it is particularly future proof extension for other possible virtualizing panels hence the wip status.

Checklist:

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Avaloniaui.net with user documentation

[ahopper]

I'm closing this as it complicates an already complex bit of code for very little general gain. Maybe the wpf source will throw up a better solution.