Blender Git Loki

UI: Bring back features for file path button

Adds back auto-completion and auto-creation (inserting a non-existing
file-path would create it) for the file path button. The second feature
was left out knowingly, but seems there are reasonable use cases for it.

We can't add these features to the button in the Python script, we have
to call into C. So using a template to do that.
Note that this is based on the old file browser code, I've copied over
the TODO comment.