Derek/ticket 379 Add generic file picker control for file inputs

[derekllanes]

Summary

This PR adds support for generic file inputs as GUI controls

The intended behavior is:

• gr.File(...) inputs for MIDI files (.mid, .midi) still behave as MIDI tracks.
• Other gr.File(...) inputs become a generic file picker control in the GUI.
• The file picker uses the model builder’s label and allowed file extensions from app.py.
• HARP does not hardcode a specific use case like NAM. The model builder controls what file types are allowed through file_types.

In the HARP UI, the new control appears as:

• a label from the model definition
• a Browse... button
• a read-only text field showing No file selected or the selected file path

What Changed

pyharp:

• Added a new HarpFilePicker component type.
• Updated generic gr.File(...) handling so non-MIDI file inputs are represented as file_picker.
• Preserved the model builder’s label, info, required, and file_types values.
• Updated the UI tester app with a generic file input for testing.

HARP:

• Added FilePickerComponentInfo to store: selected file path, required status, allowed file extensions
• Updated Model.h so file_picker inputs are parsed as GUI controls rather than input tracks.
• Added FilePickerWithLabel
• Updated ControlAreaWidget to display file picker controls alongside sliders, dropdowns, toggles, and text boxes.
• Removed the old generic file-track routing from TrackAreaWidget, since generic files should now be selected through the GUI control system.

TESTING
While testing with the updated "ui_tester" in pyharp is working. Note that extensive testing has not been done.