ExApp default app icon handling

[andrey18106]

Describe why it is important and where it will be useful

By default in various places (Apps management, TopMenu) app icon is used if exists (app/img/app.svg).
Currently we do not support automatic retrieval of app icon by its default location (app/img/app.svg), which is a proxyfied request to ExApp.

Describe your proposed solution

Implement check and usage of default app icon to be the same as for regular PHP apps.

Describe alternatives you've considered, if relevant

No response

Additional context

No response