Add display for double doors

[luca-della-vedova]

New feature implementation

Implemented feature

This PR is part of the work towards #76, no animation is done yet to keep the PR simple but display support for double doors and asymmetric doors (left_right_ratio parameter) has been added.
Future work will be based on these changes and add animation on top to fully address the feature request.

Implementation description

A new DoorBodyType enum has been added to store the mesh entities for different door elements, the animation plugin will match this enum to figure out how to actuate doors.
Double doors will now spawn two meshes with a small gap inbetween, live updates for the left_right_ratio parameter will update the scale of the doors to match the value.

[luca-della-vedova]

This is an unfortunate side effect of this PR. Changing a door from single to double will spawn an additional mesh but the system that updates the hovering and displays outlines will not be triggered since the Hovered and Selected components didn't change, meaning that the newly spawned door will not have its outline displayed until the mouse hovers on it again.

This approach works but the performance implication might not be great since now this system will be triggered much more often (Changed<Children> is fairly common).

Other approaches that I considered:

• Just triggering a change detection by doing a mutable access to the root door entity's Hovered component when a new door is spawned. Works and only a few lines but a bit hacky.
• Manually copying the Outline components when spawning a new door, probably the best option from a performance point of view but would introduce a fair bit of outline related code in the door module

[luca-della-vedova]

5461c93 is a tentative approach to optimize this, it's a new system that looks at newly spawned meshes, iterates over their parents and copies the outline components to the new mesh if any is found. It's a bit hard to benchmark performance but Added<Handle<Mesh>> should be triggered quite a bit less than Changed<Children>. Also AncestorIter goes up the parentage and should perform way less iterations than a full iteration of all descendents.

[mxgrey]

The outline copying system for new meshes is an interesting idea, but outline behavior is supposed to be based on VisualCue settings, not just simple propagation.

Overall I actually like the idea of dirtying the Hover component to trigger the recalculation. We do something similar here to update lifts using .set_changed().

[luca-della-vedova]

Changed to manual set_changed to Hovered component e2b35dd

[mxgrey]

I think the changes in this PR are good, but I thought of something related that's been irking me for a while so I left a comment on it. We can address it in this PR or in a follow up, either is fine with me.

[mxgrey]

I've always found this ratio to be kind of clunky and unintuitive. It's something we carried over from traffic editor, but I'm not sure it's the best representation of the split in doors.

I wonder if we should do a value 0.0 -> 1.0 representing what percentage of the frame width is for the "left" door, then the right door will just be 1.0 - x.