Stop using obsoleted "-moz-border-radius" and "-webkit-border-radius" pr...

Submitting fix for issue #480. (Is there a smarter way to link a pull request to an issue number?)

One thing I'm not 100% sure about is if replacing `<<border-radius 7px>>` with `<<box-shadow "inset 0 1px 0 #fff">>` in [Using Stylesheets](https://github.com/nameanyone/TiddlyWiki5/blob/bf8ad1e2947348dc1609281a35efa01714839153/editions/tw5.com/tiddlers/howtos/Using%20Stylesheets.tid#L31) makes for a good, sensible example. Please come up with a better one as you see fit.