Properties on polyline nodes

As requested on the forum, it would sometimes be useful to be able to associate arbitrary data with the nodes of a polygon.

The information could be stored as follows in the TMX format:

<object id="5" name="guard" type="NPC" x="22" y="361">
   <polyline points="-3,120 87,91 154,96 181,16 273,-1">
      <node index="2">
         <properties>
            <property name="foo" value="100"/>
            <property name="bar" value="200"/>
         </properties>
      </node>
   </polyline>
</object>

For the implementation though, it will be a little complicated because the nodes in a polygon are just floating-point coordinates, whereas everything that can have properties derives from Tiled::Object. And deriving polygon nodes from that class would of course bloat the storage requirements for a polygon, unless the properties would be stored in some other way (but that adds complexity to the Properties view).

It may be that this use-case is better solved by connecting objects (#707).

Start with the TMX example and the issue's description of polygon nodes, Tiled::Object, and the Properties view; no files or tests are named. Compare storing node properties with the connecting-objects alternative in issue #707, then establish the storage and UI approach before implementation.