Initial work to move code into Contacts.Window

Moved Contacts.Store, this broke setup related code, but that code will
live on Contacts.Window as well, so eventually will work.
ContactPane: moved into Window

Added Store as a public property. I will prefer the property being
private but GtkBuilder would not be able of setting it that way.
ListPane: moved to template file

Next thing is remove Contacts.View setting of Store in its constructor.
Contacts.View: moved into ui file

Contacts.store instant of View promoted to property. Removed classic
constructor code.
Window: moved ListPane into ui file
misc: improved ui files

ContactPane: moved options into ui file
Window: moved signals handlers internally
Window: ListPane instance made internal widget
Window: moved more content widgets internally

Content widgets are those belonging in the handling contacts interface
of Contacts, are different from the setup-widgets which are those shown
in the setup view when implemented.
[Done] button and [Edit] buttons made private.
Window: ContactPane instance made internal widget

Instance renamed to contact_pane
ContactPane: move handling edit-mode to the Window

Fix, leaving inconsistent UI state when removing a contact from ContactPane
Window: consolidating modes handling

Handling of edit-mode and selection-mode happens in only one place. This
allow more consistent switch between states.
App: removed unused 'using' statement
Window: [Select] button made private widget
AccountsList: using gobject style constructor

Further along the way this widget could be migrated to use templates
Window: added setup view widgets

Added placeholder code to handle setup showing logic and Contacts.Store signals
ListPane: removed "Other Contacts" section

Fixed: https://bugzilla.gnome.org/show_bug.cgi?id=727808
ContactPane: set Store property after construction

The point of this commit is allow the creation of an unique Store object
belonging to Contacts.App. Further along we'll make more changes in this direction
Window: replaced ListPane for loading spinner

Added regular style constructor to ListPane
App: made Store instance belong to Contacts.App

Added Window::store property, set upon constructor call