manager: update retrieve sources API

The move to newer API is to drop GtkListStore dependency on the manager.
app: remove GcalNewEventWidget, use GcalWindow ui

Since there will be no behavior on GcalNewEventWidget we can use simple
ui from GcalWindow to accomplish this.
build: remove GcalNewEventWidget related files
edit-dialog: use reccurrence_id of events

When generating the uuid of an event if the event is recurrent, you need
to take into account its rid.
window: connect signal to the right object

The ::remove_event handler was not connected to the GtkRevealer so it
was not executing itself when the notification was closed. That caused
the application never removed any events. Fixed now.
utils: add helper method for component construction
manager: updated GcalManager::create_event

Move the API to use ECalComponent instead of uids.
app: active More details button from new event popover

GcalWindow got connected to GcalManager, and pass just created component
to GcalEditDialog
GcalEditDialog gained support to handle component not committed to
manager singleton instance yet.