exo-icon-chooser-dialog: allow removing selected icon

Submitted by Jean-Philippe Fleury

Assigned to Xfce Bug Triage

Link to original bug (#8139)

Description

1. In Thunar, go to "Edit > Configure custom actions...".

2. Choose an action that already has an icon and click on the "Edit" button.

3. Try to remove the icon, so the action would no longer have one.

Actual result: it's not possible. We only can choose another icon.

Expected result: we should be able to remove the icon, for example by choosing an item "No icon" in the drop-down list.

Version: 0.12.11