Currently, when selecting a file path using the file browser opened from the
node socket, there is no filter active making it harder than necessary to find
the correct file. This patch adds the proper filter. Something similar is done
when using e.g. the gltf import from the menu.
Supporting this requires changes in a bunch of places:
* `StringPropertyRNA` now has a callback that returns the file path pattern.
This has to be a callback, because the same property is used on all file path
sockets, but the valid extension depends on the node.
* The string socket declaration also has the optional path pattern. This can be
set in the node declarations.
Pull Request: https://projects.blender.org/blender/blender/pulls/134931
e05ef4c838