Overrides: show main app file

Added a startup hook that shows the main application file in case no files are opened.

Created a public API for embedders, added function buildInspectorUrl().

/cc: @Schoonology please review.

Note: the first implementation was based on `#show={fileUrl}` parameter, therefore it was highly desirable to add a public API for embedders. After I have reworked the feature to always open the main file, the API is not that important, but it's still nice to have it.