Summary:
This is an attempt to unify existing Calendar&Tasks and Contacts resources into
a single Groupware resource. At some point, hopefully, GMail support could be also
added here (see task {T646} and {T9422}).

Various "subresources" (Calendar, Tasks and Contacts) are implemented as subclasses of `GenericHandler`,
which is a basic `Akonadi::ResourceBase` interface. The resource decides which `Handler` it should call
by looking at mimetypes. `Handlers` are `friends` of `GoogleResource`, so they can call its callbacks
(like `itemsRetrieved()`) as needed. This was done primarily to separate logic of different subresources;
this might be not the best solution, I'm open to suggestions.

This patch also reworks the settings dialog & relevant code.
The dialog is now using `.ui` file. The "account picker" is gone, as it's no longer needed;
instead, a single "Configure..." button is added which invokes the auth process.

It also implements "last sync token" API ({T647}) for calendar incremental updates. Without this API,
event moving between calendars were not handled properly (i.e. event was not removed from the "source" calendar).

Work to be done:
 # KAccounts integration. Need to be able to `disable` various `subresources` on demand, and determine auth `scopes` based on that.
 # GMail integration. Need to somehow adopt `ImapResourceBase` / `ResourceState` scheme, and merge it with current `Handlers` scheme.
 # Add `Akonadi::Tag` support for Contacts. Tags seem to be more appropriate than having bunch of virtual collections, but this might require some changes inside KAddressBook.

Test Plan:
Here's a comprehensive list of what was tested and what issues were discovered.
 # Adding event locally
 # Changing event locally
 # Removing event locally
 # Moving events between calendars locally
 # **Adding calendar locally**
   - new calendar is added as a virtual collection, cannot add events there afterwards; probably a KOrganizer issue
   - color of newly added calendar is not known, google just don't return it to us
 # Removing calendar locally
 # Changing calendar locally

 # Adding event remotely
 # Changing event remotely
 # Removing event remotely
 # Moving events between calendars remotely
 # Adding calendar remotely
 # Removing calendar remotely

 # Adding task&subtask remotely
 # Changing task locally
 # Removing task locally
 # Removing a task with subtasks locally (subtasks go to the upper level)
 # **Adding/removing tasklist locally**
   - wasn't tested, but should work. KOrganizer just don't know how to add a tasklist, it adds a calendar by default (probably it sees no difference between them...)
 # Changing tasklist locally

 # Adding task&subtask remotely
 # Changing task remotely
 # Removing task remotely
 # Adding tasklist remotely (it is not subscribed automatically, however, so user have to go to account settings and enable it)
 # Changing tasklist remotely
 # Removing tasklist remotely

 # Adding contact (including photo) locally, both inside "My Contacts" and "Other Contacts" groups
 # Moving contact between "My Contacts" and "Other Contacts" groups
 # Chaging contact (including photo) locally
 # Removing contat locally
 # Adding contact to contact group locally
   - {D28432} required, otherwise "Link Item" gets silently ignored
 # **Removing contact from contact group locally**
   - wasn't tested, since I've found to UI to "Unlink Item" :(

 # Adding contact (including photo) remotely
 # Changing contact (including photo) remotely
 # Removing contact remotely
 # Adding contact to contact group remotely
 # **Removing contact from contact group remotely**
   - doesn't work: need an easy way to fetch all collections we're linked to (so we know which UnlinkJobs we should start)

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28560