Implement a Sharing Portal

### Problem

This was discussed over email with @marbetschar.

I'm explaining this using an example. Lets say someone is reading an article on some app, say Communique and wants to save that article as a reminder in elementary Tasks. Currently, there is no way for Communique to save a reminder to Tasks. The user will have to manually open Tasks and create the task themselves and save it to a project.

Other apps can also benefit from this functionality.

### Proposal

Implement some sort of Sharing Portal which is capable of receiving different types of information and forwarding that to the respective responsible apps.

### Prior Art (Optional)

#### macOS

#### iOS