(minor) line: Add support for setting x or y of start/end

You can now set only one of x or y for the start and end
properties, like this:
line.start.x = 50;
(minor) animation: Add support for nested properties

Nested properties will now work, like this:
line.animate({
  start: { x: 50, y: 30 },
}, { duration: 500 });