Rewrite Point mark

[luucvanderzee]

Currently the Point mark is almost identical to the Symbol mark. @GraceGSy and @atepoorthuis suggested completely removing the Point mark and replacing its usage by the Symbol everywhere, but I think it is nice to be able to do vgg-point in the template because it is more explicit. So I suggest we refactor Point.vue to basically a wrapper around Symbol.vue, that only does to things:

• Render a Symbol of type 'Point'
• Expose a :radius prop will be multiplied by 2 and passed to the Symbol's size

This way we can still use vgg-point in the template AND we don't have double code :)