Issue #3560377: Content outline: Allow sections to be expanded and shortened - move InitialTextPlugin into it's own file and add tests

Issue #3560377: Content outline: Allow sections to be expanded and shortened - move InitialTextPlugin into it's own file and add tests

Testing instructions

• In a new draft go to the last tab, verify all works as expected
• Enter some content into the draft and then close Conductor
• Reload, then re-open conductor to load the draft. Load the last tab and verify that a previously saved draft is restored in the main content area and editing works as expected