Fix demo
Very in-progress attempt at merging in hwthomas' NPlot fork

* He handles multi-platform interactions appropriately
Remove interactions registry
Basically working integration of hwthomas' NPlot fork, WinForms part
Rename WinFormsPlotSurface2D to more accurate PlotControl
Add untested implementation of Gtk# widget
Create unified demo for WinForms and GtkSharp
Use .NET 4.0 Client Framework for DemoLib to match other libraries
Clean up Demo project files a bit
Shut down Gtk# demo cleanly
Consolidate ReDraw and Refresh in InteractivePlotSurface2D
Fix bug that wasn't drawing interactions in Gtk#
Add alternative clip computation and some debugging info to Gtk# drawing
Add Gtk# version of imperative mode, and simple examples of using it
Refactor and reorganize.
Update README