ID 394556 - [C] Panel browser: Large panel selection causes drop in performance

Problem Summary

Flix becomes slow and unresponsive when selecting a large amount of panels. 

Steps to reproduce

1) Create/open a revision with 300+ panels. 
2) Select all panels.  
3) Flix Client is unresponsive, there is no progress bar to show what it is doing, there is no indication of whether or not panels are being selected during this unresponsiveness, and it is not possible to use any Panel Tools.

Expected outcome

Flix should be responsive and Panel Tools should work as normal. 

Actual outcome

Flix becomes very slow and Panel Tools are unresponsive and/or very slow. This also causes scrolling through the Panel Browser to lag.