Recursively search for files to open

[shilorigins]

What's the problem this feature will solve?

Displays can only open files located immediately in the calling display's directory, the cwd, and a few preset directories such as $PYDM_DISPLAYS_PATH. There is an arg extra_paths in the code, but I don't see how to access this arg from a display, and using it would require hard-coding the extra paths in advance. It would be useful for displays to be able to open files that are located anywhere under the searched directories.

Describe the solution you'd like

PyDM should recursively search directories when opening files, so that any display can open any other display that is located under the search directories.

Additional context