Create help (tutorial) page

[lilliputten]

Potential topics to explain in the document:

• the behavior and effects of controls on the settings and properties panel
• node selection and dragging
• chart toolbar: PNG export and 'reset' buttons

Something else?

The document should contain screenshots of the explaining ui elements.

Help page should be open as a popup or as a document in the main application area.

Alternative way: to implement interactive tutorial, where explanation popups should be open by clicking on specific ui element/area.

[lilliputten]

Implemented in PR #23.