Rapid clicks on "Load" creates multiple pools

[philrz]

Repro is with Zui commit cf615ef.

This issue has a feel similar to #2940. It may be worth addressing both at the same time and thinking about if there might be other spots in the app with exposure to the same kind of problem.

Repro steps shown in the attached video:

1. Drag a file into Zui to bring up the Preview & Load screen
2. Click the Load button multiple times quickly

A separate pool is created for each click. If a user happened to double-click on the button, I doubt this is the behavior they'd expect.

Repro.mp4