Reintroduce GTG tasks DBus interface with Gio

[Neui]

This is Work In Progress! Fixes #277. View the commits for some more information.

Some stuff is missing:

• [x] Properly implement ModifyTasks, AddTasks
  • I'll make it simple and don't implement adding tasks without knowing their final IDs
• [ ] Better error handling
• [ ] Test it
• [ ] Document the interface introspection XML data/dbus/org.gnome.GTG.Tasks.xml
• [ ] Implement recurring fields

Decisions:

• Possibly offer pagination so client don't pull thousands of tasks at once?
• Possibly use an tuple for more common fields (title, text, dates, status, and the dictionary for future stuff)

Future:

• Check about performance with many tasks, possibly outsource it to an separate thread

[diegogangl]

Can I leave a wish here? It'd be nice to have a dbus interface to add tasks with the quick add syntax (just passing a string basically). That way we can implement a mini-app to quickly add tasks from a global shortcut (I could swear there was an issue about that but I can't find it now)

[gwutz]

In order to reactivate the GNote plugin i think its necessary to have a dedicated Bus method to add a single Task withouth knowing the resulting ID. I would love to reactivate this interaction as it fits perfectly the flow "Taking Notes -> abstracting tasks from these notes into GTG"

[khurshid-alam]

@diegogangl I was looking for to use gtg plugin for kupfer launcher with newer gtg where tasks can be seached, opened and created. It used to work with older version. But found out dbus interface doesn't exists any more. This is how it was it looked.

Is this the correct branch? Any way it can be revived with old methods ? Like OpenTaskEditor(tid), OpenTask(tid, description) ? @Neui ?

[nekohayo]

Is this directly related / a prerequisite to #676?

In any case, needs to be rebased for 0.7.

[Neui]

Is this directly related / a prerequisite to #676?

No.