manager: fix clients' hash construction

free_manager_unit should have been removed from clients construction previously
manager: remove internal query

The internal query is not needed since that's handled now by the
ECalDataModel instance
manager: remove internal date range

This is not needed since the range is kept by ECalDataModel instance
manager: fix remove source from ECalDataModel

When a source is removed should be removed from the internal
ECalDataModel instance as well
style: add entry class to notes textview
build: stop complaints from gcc

Gcc complaints this variable may be used uninitialized
build: add GcalSubscriber abstract class

GcalSubscriber abstract class is meant for handling the implementation
of ECalDataModelSubscriber interface. The code in each vfunc implemented
is the same for each view, then it makes sense to have a class holding
these implementations and make every views descend from it. This class
sits in the middle between GtkContainer and each of the views.
month-view: makes a descendant of GcalSubscriber

By making GcalMonthView a descendat of GcalSubscriber it gets
implementation for each of the methods on ECalDataModel subscriber and
utomatically gains the functionality of adding and removing events widgets
year-view: makes a descendant of GcalSubscriber
view iface: remove unused will_add_event

ECalDataModelSubscriber interface notify each view only about the events
beloging to the subscribed range so there's no need of checking if the
event widget will be added.
The safety check of not adding the same widget twice its stil being made
on each GtkContainer::add implementation.
month-view: remove FIXME

Subscription is already being made. Remove reminder.
window: remove FIXME comment

edit_dialog_closed already hides the widget. Reminder is not needed anymore
window: replace New Event with Today button

As Lapo rightfully explain, providing a button there for creating a new
event is way to prominently and might divert users of the intended click
on a cell behavior to create an event.
New event will be also accessible from the hamburger icon on the left of
the header-bar and Ctrl-N key shortcut.
Also as Lapo suggestion Today button fall better on the header bar than
between the navigation buttons.
window: added New Event item to menu button

Updated label on app-menu action.
view iface: another round at cleaning

Remove unused methods laying there for a long time now.
view iface: remove more unused methods

GcalView::draw_event overlapped a bit with GcalView::will_add_event, so
the same reasons for removing one stand for the other.
GcalView::clear was a hack for the composited GcalDayView which has
gone, and was a bad experiment.