source-manager: Simplify tracking of GOA accounts
tracker-controller: React to changes in GOA accounts
Use the correct typecast
source-manager: Check GoaAccount:photos-disabled when refreshing

When the "Photos" switch is flipped in the Online Accounts panel
GoaClient::account-changed is emitted twice. Once each for:
 - org.freedesktop.DBus.Properties.PropertiesChanged
 - org.freedesktop.DBus.ObjectManager.Interfaces[Added|Removed]

We should filter based on GoaAccount:photos-disabled too, so that we
catch the addition or removal of an account as soon as possible.