Add Templates functionality to JDD.

The use-case is that sometimes the user needs to have multiple pages with similar structure, for example - each article starts with an image and a heading with some alt-text. Instead of entering it by hand each time, they could turn a component from already existing document into a Template. A Template would consist of:

• the template name
• the JDD component name
• the values for that JDD component's arguments

The templates then would appear in the dropdown for choosing the component in JDD editor, in a separate section at the bottom of the dropdown list. When picked, a new segment is added to the document in jdd editor, with the values from the template pre-filled (and editable).

This task might involve the JDD Editor repository more than the core JDD one.