Fixed extents to respect direction of axes; more granular extents methods; misc

Moved main into form so new devs have only one entry-point to start.
Moved extents methods to extension methods on their respective classes but with facades on the Chart class for easy access.
Implemented extents for second X/Y axes if present (no demo though).
Added GUI demos for extents and plotting with reversed axes.

Hope these are all agreeable changes. I don't feel too strongly about having main within the form, I just think that it makes the demo project even easier to get started with. Oh, I also removed all of the unused toolbar button images.