Urdf import, export and workcell editor fixes

[luca-della-vedova]

New feature implementation

Implemented feature

Based on #155 by @audrow, extends urdf export to also export joint and inertial data, as well as add a urdf import feature to convert URDF into a Workcell object. Having both allows adding an integration test that performs a roundtrip conversion in rmf_site_format and makes sure that the files match.
I am also refining various parts of the workcell editor mode and fix issues I found along the way. Will change it to in review and add a more detailed description of the changes once it's ready.

Implementation description

• Removed rapier dependency, it was meant to be used for physics simulation but it was effectively never used, we can reintroduce it when needed. Saves us some compile time and runtime overhead.
• Fix mesh primitives that were not selectable.
• Add a system that "explodes" SDFs imported in the workcell editor mode to unpack all the visuals and collisions contained in them.
• Fix parenthood of models that was not set properly.
• Add joint components / data to the Workcell structure, including conversion back and forth from urdf.
• Add a basic joint inspection widget, as well as a button to create a joint when two frames are connected directly in the hierarchy without a joint inbetween.
• Update the mesh primitive inspector widget to only have read-only labels, until we implement systems to update primitives based on sizes / type.
• Add a keybinding (Ctrl-E) to export to URDF. For the moment I didn't add a menu button since that would have needed adding a state dependent logic to the top menu bar.
• Add inertial data to the Workcell format, including saving and loading to a site. No inspector is implemented yet though.
• Added a temporary workaround system that replaces NameInSite components for NameInWorkcell if the spawned object is a child of a Workcell. This will probably be removed in the future by having a NameInGroup component that unifies the two.
• Add support for opening urdf files through the Open File window.

[luca-della-vedova]

There are a few rough edges here and there but this is "stable enough" that it should be pretty much OK.

[mxgrey]

I know this has been baking for quite a while and much of the implementation details are subject to change as we obtain better foundational tools in Bevy, so I'm only doing a cursory review to unblock this.

I only noticed a few things that seem off, and they're relatively minor, so I've opened issues #198 #199 #200.