Blender Git Loki

Nodes: initial support for socket tooltips

This adds initial limited support for socket tooltips. It's limited
in a couple of ways for now:
* Only works when hovering over the socket shape, not when hovering
over the value in the socket.
* Only works for built-in nodes that already use the new node
declaration system. This can later be extended to support pynodes.

Those limitations are well worth it for now, given that the
implementation is quite simple and the impact on usability is quite
large. More complex updates to the layout system, that would allow
showing socket tooltips in the nodes, can be done later. With the
current implementation we can at least start writing tooltips for
geometry nodes now.

This commit already adds tooltips for the Cylinder node as an example.

Differential Revision: https://developer.blender.org/D12607