Add a special prompt to add tags to an email

[micz]

Add a special prompt, extensible by the user, to add tags to an email.

The feature should be activated by the user and adds a fixed button to the dynamic menu (accessible also with a number, like "0").

This feature should be available only when using an API integration, at least in the first release.

To do:

• Add an option to activate the feature
• Define a special default prompt
• Add a tool to extend the prompt
• Add the special item to the dynamic menu (only when reading an email)