Feature request: spline objects

Add spline (Bézier and possibly Catmull-Rom) object types and drawing. When exported, export the control points of the curve. Example: I have a game where I want a boat to arrive into and exit from a port using a non-linear path. It would be amazing to just draw this spline path into Tiled, then read those control points in my app to easily construct a movement path.

No files, tests, or entry points are named. Begin by locating the existing object types, drawing tools, and export handling in Tiled; done means Bézier and/or Catmull-Rom spline objects can be drawn and their control points are exported for use by a game.

Written by the indexing model from the issue text.