Improve theme preference selection

[msujew]

What it does

Closes #11672

Changes the preferences for color theme and icon theme to use an enum. In order to facilitate label displaying and changing the associated preference schemas during runtime some more changes had to be performed:

1. Support for enumItemLabels in PreferenceSelectInput
2. Add updateSchemaProperty method to PreferenceSchemaProvider that triggers the associated change event.
3. Creating and storing keys for each preference while rendering. Allows us to identify which preferences have changed. The previous method only could tell whether a preference was removed/added.

How to test

1. Open the preferences and search for the file/color theme preference
2. It should be a select component and display all currently displayed themes
3. Changing to another file/color theme applies the change as expected
4. Installing a new file/color theme using plugins updates the preference widget and allows to select the newly installed theme

Note that uninstalling color themes doesn't work correctly due to #11676.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM 👍

• both the color and icon theme use a select component for theming similarly to vscode
• changing color and icon themes work as expected
• the list is correctly sorted, and the default value is identified
• resetting the color and icon themes work as expected (the list is correctly updated)
• installing new themes are listed in the dropdown