class_docstr: This class provides the UI for the file collection display
  
  The UI has a browse button, a hide checkbox and a tree control.
  
  Critical attributes:
  
  self.walks_in_progress - this is a dictionary of keys to directory walks
                           and metadata fetches that are happening in the
                           background. The value of the dictionary entry
                           is the function to call to stop the search.
  
                           There's a completion callback that's called to
                           remove an entry from the dictionary. When the
                           dictionary size reaches zero, the stop and pause
                           buttons are disabled.
  
  self.modpath_to_item - a modpath is a collection of path parts to some file
                           handled by the controller. There's a tree item
                           for every modpath in this dictionary and the
                           dictionary can be used for fast lookup of the
                           item without traversing the entire tree.
code: def on_tree_key_down(self, event):
      LOGGER.debug("On tree key down")
      key = event.GetKeyCode()
      if key == wx.WXK_DELETE:
          self.on_delete_selected(event)
  
imports: import logging
  import os
  import sys
  import uuid
  import wx
  from cellprofiler_core.preferences import report_progress
  from cellprofiler_core.setting import FileCollectionDisplay
  from ..pipeline import Pipeline
  from ..utilities.module_view import edit_control_name
  from ...icons import get_builtin_image
label_desc: Use image node_id 18 for calling the FileCollectionDisplayController obj's underlying member method code with example usage: obj.on_tree_key_down(event) without return types