Unable to edit "When" field in keyboard shortcuts editor

[dsseng]

Description

When trying to edit keyboard shortcuts from the UI I can't edit When. I can only do so after I create a custom keybinding and edit it in JSON.
In VS Code a context menu pops up when user right-clicks on a keybinding, like this:

2nd item from the and means Change When expression, this is the option I couldn't find in Theia.

Reproduction Steps

1. Open Keyboard Shortcuts (Ctrl+Alt+,)
2. Try to find any button or right-click on a keybinding item
3. Find no way to set its When condition

OS and Theia version:
OS: Ubuntu 19.10
Theia: master

Diagnostics:

[vince-fugnitto]

Closing in favor of #7582