month-view: fix insert ordered widgets

The head of the list might changed when ordering the list at insertion,
then, some events was getting lost
edit-dialog: don't open with summary entry empty
gcal-view: signal the opening of edit-dialog directly
month-view: emit ::create-event-detailed

Emit this signal from the "Add new event..." button of overflow popover,
the reason is mainly aesthetics since it won't look good showing one
popover after another.
window: use GcalView::create-event-detailed signal
month-view: monitor click only on overflow indicator
event-widget: remove unused CSS tag name

Since Gtk+ parses automatically widgets' name, there's no need of custom class
app: settings gained "disabled-sources" key

This key will keep a list of the uid of the sources marked as disabled
in the interface in order to keep the same state between runs of the application
manager: hold a reference of the application's settings

We need to keep on the settings the list of the disabled sources
Moved code from ::init vfunc to ::constructed vfunc in order to wait for
construction properties to be set
manager: use "disabled-sources" settings
window: unmark disabled sources checkbox

Calendars selector menu needs to know which source is enable to properly
mark the checkbox
Revert "edit-dialog: update sources when their readonly state changes"

This reverts commit 31fa4e8e06e9a2b74d9e73f842dd9ca07a1aafbd.
manager: make ::get_sources retrieve only enabled sources
manager: emit ::source-activated on enabled sources

The ::source-activated signal has no much sense right now, I'm leaving
it around, for a while, in case some use case pop up. Otherwise in time,
it will go.
build: remove unusued code

Since we don't build treeview anymore using cell renderer, this code is
not needed.
manager: get all connected sources

Added ::get_sources_connected API to retrieve a list of every source the
application has connected, no matter whether they are enabled or not.
Use ::get_sources_connected on calendars menu in GcalWindow to enable
disabled sources.