Show a validation warning when someone sets a `setColorScheme` action
with an invalid scheme
In the setting validation phase, scan all commands for all the "set
color scheme" actions, and check each of them has a valid scheme. If any
of them has an invalid scheme name, raise a warning. Do not check
iterable commands that will be expanded to valid color schemes.
## Validation Steps Performed
- Added tests to LocalTests_SettingsModel
- Manual tests, add commands to settings.json with invalid color scheme
and check the warning pops up. Try simple and nested commands.
Closes#7221
f8edcf57bd