B&I: Fix non B&I trays disappearing when B&I is open
B&I: Fix non B&I trays disappearing when B&I is open

RCEWrapper needs to understand that there could be multiple trays open

fixes MAT-708
flag=buttons_and_icons_root_account

test plan
1. Find an RCE
2. Insert -> Buttons and Icons -> Create Button and Icon
3. Insert -> Buttons and Icons -> Saved Buttons and Icons
   CONFIRM that the saved B&I tray opens and doesn't disapper
4. Refresh the page
5. Repeat steps 1-3 except at step 3, open one of the other trays
   (e.g. Insert -> Document -> Course Documents) instead

Note that the B&I tray will remain open beneath the second opened tray.
This is not ideal and requires some future refactoring to address.

Change-Id: Iaf3bebb29fe2803b67a3bf2a81d7be582401926f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/286406
Reviewed-by: Juan Chavez <8fc9c8a746010dcf1babfbe57a3b7ae7798e6eeb@instructure.com>
QA-Review: Juan Chavez <8fc9c8a746010dcf1babfbe57a3b7ae7798e6eeb@instructure.com>
Product-Review: David Lyons <c345b6495d82f574bde007a1165ad87fad2909cd@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms