[enhancement] As a downstream consumer, I want to reuse the preferences editor and feed it with custom YAML content and scheme

[kittaakos]

Feature Description:

The new preferences editor is just great; I want to reuse its functionality and UI. As the ID of the preferences-widget suggests, maybe it can be converted into a more generic, settings widget:

theia/packages/preferences/src/browser/views/preference-widget.tsx

Line 30 in 3eb0678

static readonly ID = 'settings_widget';

Pipe dream:

• I have a YAML file, its content is available as a Theia Resource on the frontend,
• I have a scheme for my YAML file,
• I would like to open and edit my YAML file through the currently available preferences widget.