Author-friendly add an app to a rich text block

### As a content editor I want the ability to add an app in a step to be user friendly, so that I can easily use Joplin.

**Is your feature request related to a problem? Please describe.**
In #2666, we made it so that a user can add an app in a step block. Our temporary solution is to add a "code" block to an existing step, use some fancy keyword to mean "hey Janis, we want an app here."

Now let's make it user-friendly!

**Describe the solution you'd like**
An app picker: Possibly add a button to the rich text menu that allows you to select an app.

**Describe alternatives you've considered**

**Additional context**

Start by locating the rich text block and the existing step-block handling described in issue #2666. Define the app-picker interaction and integration point, then verify that an editor can select an app without using the temporary code-block keyword.

Written by the indexing model from the issue text.