vsx-registry: display incompatible extensions as disabled

[vince-fugnitto]

Feature Description:

Rather than completely hide incompatible extensions (non api-compatible) in the extensions-view, we can display them as disabled. At the moment there may be confusion for end users when they see extensions available from open-vsx and not in the application.

We can mark them as disabled in the tree:

• visually disabled (such as greyed out),
• hide the install functionality from them.

Related conversation: eclipse-theia/theia-ide#96

[Jeeppler]

Yes, I 100% agree.

[vince-fugnitto]

In VS Code they introduced the idea of deprecated extensions which would clash with this approach (marking them as disabled). If anyone is interested in still rendering extensions which are incompatible they might need to find a new way to visually mark them as such.