Allow developers to send messages to the users

[clochix]

We should allow developers to send short messages to their users.

Use case:

• instruct users to reinstall the application because its name or URL has changed (which may lead to future issues);
• advise users to update the application because a security issue has been found;
• …

We may add to application manifest a field with a messages related to application versions, for example: '<0.9.3': 'please update'. When Home looks for the availability of a new version, it will also fetch this messages and display them in the update notification.

[jsilvestre]

This is a good idea!