Prevent event handler from being triggered twice

[eloquence]

Description

Fixes #1217

Test Plan

Must be tested in Qubes or with local modifications, as clicking files in a development environment will have no effect.

1. Run the SecureDrop client from this branch
2. Connect to a server with file submissions
3. Download a file submission and attempt to open it

• Observe that only a single diposable VM is opened
• Observe that print/export actions still start dialogs as expected

4. Resize the client window to its minimum size
5. Observe that Conversation View cut off when scaling down client #1061 and Resizable speech bubbles and file widgets #1204 are still fixed and that there are no regressions in resize behavior.

Checklist

• I have tested these changes in the appropriate Qubes environment
• No update to the AppArmor profile is required for these changes
• No database schema changes are needed

[eloquence]

In testing in Qubes all LGTM; I did observe #1218 again, but I believe that's unrelated and seems to occur also in the recent signed release.

[sssoleileraaa]

lgtm and makes sense why this was triggering twice :)