Refine `talondoc autogen` to create entries for known objects

[wenkokke]

The files generated by talondoc autogen should generate:

• A header for the file
• All commands defined in the file
• A header for each module
  • All actions declared
  • All captures declared
  • All lists declared
  • All modes declared
  • All settings declared
  • All tags declared
• A header for each context
  • All actions overridden
  • All captures overridden
  • All lists overridden
  • All modes overridden
  • All settings overridden
  • All tags overridden

Furthermore, each header and subheader should only be generated if it'd be non-empty.

[wenkokke]

Fixed by #153.