Delay initialization of svg-edit until it is safe to do so

This isn't ready to merge yet.

This is one way to solve a problem that happens in Firefox. If the iframe containing svg-edit is loaded when it is invsibile `display:none` then one of the svg tests fails that svg-edit is trying to run. This approach stops the automatic loading of several parts of svg-edit. And then leaves it up to the user to call `svgEdit.init()` when it is safe to do so.

This approach isn't really viable to merge upstream. Because it will break existing users of svg-edit. But it does appear to make things work in Firefox when used correctly.

I want to explore another option for handling this which which would let svg-edit initialize when it is invisible, but then certain parts of that initialization could be re-run if a user thinks it is good to do so.