compatCLAdd, compatCLToggle: fix & add comment

The old className-based implementation for the compatCL functions never
actually worked: the className attribute for SVG doesn't play nicely
with others, possibly due to being an instance of SVGAnimatedString.

Instead, we use className.baseValue, which should always behave in a
string-like fashion, and appears to function correctly.
Fix right-click on IE

This moves the oncontextmenu listener from the SVG element to its DIV
container. This seems to work on IE, FF, and Chrome, whereas the old
approach only worked on the latter two.
Set clicked class unconditionally in handler

When the clicked/flagged/bomb class cleanup was made, setting the class
to 'clicked' was made conditional on the poly not being a bomb. This
broke some things, and the class changes appear to have not worked 100%
correctly anyhow.

This commit makes the clicked class uncoditional again, and uses CSS
selectors to make it look right.
CSS: Corbel -> Arial

Corbel digits look terrible in boxes.