Sync projects as headings

[olexs]

I use projects extensively in my Things setup, and wanted to have an option to use projects as subheadings when syncing tasks, in the same way as areas are used. The new feature is implemented as an option, disabled by default, so the previous default behavior (group by areas only) is maintained unless the user explicitly enables the option.

The new behaviour is as follows:

• "Include project" disabled (default), same behaviour as before the new feature: -- Task has NO AREA: task is directly listed under "Logbook" -- Task has AN AREA: task is listed under the "Area" subheading, 1 level below "Logbook"
• "Include project" enabled: -- Task has NO AREA and NO PROJECT: task is directly listed under "Logbook" -- Task has AN AREA and NO PROJECT: task is listed under the "Area" subheading, 1 level below "Logbook" -- Task has AN AREA and A PROJECT: task is listed under the "Area" subheading (area has priority over project) -- Task has NO AREA and A PROJECT: task is listed under the "Project" subheading, 1 level below "Logbook"

Relates to #26. The desired implementations described there are a bit more complex, but should also be doable based on my changes.

Additionally, I added an option to pad the area/project subheadings with a preceding newline. Same thing, disabled by default to maintain previous behavior on update.