fix pagination in tinymce image embed widget

this is not a full fix, because the TreeView is assuming a certain type
of containing element to set up it's pagination listener.  however,
restores functionality to how it was working before we genericized this
component.  a full fix is still needed.

fixes CNVS-17617

test plan:
- create a course with 11 folders in it
- go to a page with a tinymce editor (like editing a wiki page)
- click on the image embed
- click on the canvas tab
- scroll the list of folders
- all 11 should be shown

Change-Id: Ifbfe51df13ea45737a9ee4b592fda07d32697c7e
Reviewed-on: https://gerrit.instructure.com/46308
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>