manager: overwrite constructed vfunc is not needed

Since GcalManager does not have any properties or use in way GObject's
initialization system there's no need of having initialization code
split between vfunc _init and vfunc _contructed.
build: added ECalDataModel related files

Implements an ECalDataModel object from evolution courtesy of Milan Crha
ECalDataModel hides the details of events retrieving from
calendars. Provides a time range and a subscriber system allowing
objects implements the necessary callbacks to be notified of events
happening on the selected time range.
manager: use ECalDataModel to handle events

Initial work. Code is marked all over the place with comments
GcalManager:set_subscriber will set a view and a new range in the
internal ECalDataModel instance.
More cleaning is needed, along the way unneeded methods will be removed
build: removed unused/faulty code

This cleaning is made in order to merge this branch into master as soon
as possible.
The implementation of GcalDayView is planned to go on the same lines as
GcalWeekView but for two days alone, so much of the code of the latter
could be reused. The approach of using shallow container and kinda
layout manager objects inside GcalDayView didn't worked that well for my
expertise at this time.
docs: updated HACKING style guide
view: added :event-activated signal

Since the events widget are handled entirely inside GcalView
implementations (aka the views), we need a way to hook event-widget
activation with GcalWindow which is handling the dialogs (as expected).
This signal serve as a bridge between those two objects.
view: gained a :manager property

Forcing every GcalView implementing class to keep a reference (actually
it is a simple pointer) to GcalManager instance coming from the application.
views: implemented :manager property

As explained before, this is only intended to keep a reference to the
GcalManager instance of the application.
Misc: fixed some documentation and whitespace issues
window: removed unused code

Events related signals from GcalManager had been removed previously,
those handlers aren't needed anymore.
Second, there's no need to inform GcalManager of the new range since
every view will do so internally. Also, there's no need of removing
children widgets from the active view, since each view will do so on its
own by implementing ECalDataMoelSubscriber::components_removed vfunc.
window: add :manager property to construction

Since a pointer to the GcalManager instance it's needed at construction
time and GtkWindow::application property is not set as construction
property. We need to add this property to have a reference to pass to
the instantiated views.
Just like the views, this isn't ref/copied or anything, it's just a
dummy pointer. The application objects is the respnsible of handling
GcalManager instance initialization and destruction.
month-view: add ECalDataModelSubscriber iface impl

Initial work implementing ECalDataModelSubscriber interface. As
explained before, on each set of GcalView:active-date property the range
the view its subscribing gets updated. Allowing this to obtain
notification through ECalDataModelSubscriber::components_added and
ECalDataModelSubscriber::components_removed vfuncs.
window: hook GcalView:event-activated signal