Big Wpf updates

Hey Vivek,

Here's some big WPF updates.  Namely, my app that previously would not run on WPF is now fully functional.  There's now a huge performance boost for large drawable controls.  The tiling mechanism has been moved out of the Scrollable (which used VirtualCanvas for tiling) into the DrawableHandler itself.  There is no longer any flickering when scrolling/resizing which was a big issue for my app.

Other notable things included here:
- WPF Draw/Fill methods now clone the pen/brush so that any changes to the pen/brush will not affect the final rendering.  This comes at a cost of performance (not much it seems), but is the only way to get the same behaviour as the other toolkits.
- Fixes for controls that want to handle both a single click and double click events
- Fixes for Open/SaveFileDialog initial directory
- Window.UseLayoutRounding is now set to true to ensure drawing tiles (and other controls) lay on pixel boundaries (which oddly SnapsToDevicePixels does not actually do)
- Padding fixes for DockLayout

Cheers!
Curtis.