docs: update code style
manager: initial work on new design

GcalManager new design includes:
1. remove deprecated methods from eds
2. remove event cache
3. remove the need for keeping date range logic
4. reorder code according to HACKING code style document

This changes will cause some other changes among the application. While
this commit build just fine, work is missing yet. Next I will fix
GcalEventWigdet and the rest of the affected components.
manager: more work into migration

1. Some code style updated
2. Remove unneeded function
manager: migrate :objects-added signal

GcalManager:objects-added signal pass a list of struct containing
ECalComponents for the event widgets. This ECalComponent instance will
be owned and released by the event widget. Look at: GcalEventWidget::finalize
view: add :will_add_event vfunc and stub methods

Note: this commit probably won't build, but this is the right order of
keeping history.
event-widget: add internal #ECalComponent object

The changes made here are minimal to get this building. GcalEventWidget
needs a new rethink and rework to accommodate to those new internal
attributes, which replaces some of the old ones.
window: update GcalManager:objects-added handler
event-widget: set :uuid property from data
month-view: implement GcalView::wil_add_event vfunc
manager: update functions docs
manager: enable sources by default
event-widget: add method to pass its internal data
utils: fixed debug method
manager: remove get_event* methods

Now those values are accesible from inside GcalEventWidget and
GcalEditDialog using its internals copies of ECalComponent of the event
they represent
window: migrated to match latest GcalManager API

Reorganized some code as mentioned on HACKING code style file
Removed unused methods
Started migration on new EditDialog flow to match latest GcalManager API
Updated GcalManager:events-modified handler to match lastet GcalManager
API
edit-dialog: added stub code

The code added make the dialog show up. Probably anything else works,
but it's start to keep the project building while GcalManager migration
is finished and new GcalEditDialog is built.