Cannot uninstall color theme plugins

[msujew]

Bug Description:

I didn't dive too deep into this issue, but noticed it while working on #11672. When uninstalling a color theme, the frontend contributions from that plugin aren't disposed at all. When reloading the application, the monaco theming service still thinks these color themes exist, and stores them in the indexed DB. After reloading the plugin has been disposed, but since the indexed DB still contains the themes, the theming service loads them from the indexed DB, making it effectively impossible to uninstall color themes.

Steps to Reproduce:

1. Install a color theme
2. Uninstall it
3. Confirm that the color themes are still available for selection, even after reloading the application

Additional Information

• Operating System: Windows
• Theia Version: 1.29.0

[vince-fugnitto]

I meant to open this a while ago but forgot, thanks!