ConfigFile is a GLib.KeyFile-like class (and is backed by a KeyFile) that
nonetheless provides convenient a high-level API for getting and setting
grouped config values and asynchronous loading and saving.
* src/engine/util/util-config-file.vala: New ConfigFile class.
* src/engine/api/geary-service-information.vala (ServiceInformation):
Require ConfigFile groups rather than KeyFile instances for loading and
saving. Update subclasses and unit tests.
* src/client/accounts/account-manager.vala (AccountManager): Move generic
account key consts here from Config. Instead of using KeyFile directly,
use ConfigFile groups for loading and saving settings. Rename load and
save methods to be a bit more consistent with the class's role, and
make save_account() throw its errors so they can be reported to the
user. Update call sites.
* src/client/accounts/local-service-information.vala
(LocalServiceInformation): Move service-specific config key consts
here, use new ConfigFile groups for loading and saving.
* src/engine/api/geary-config.vala: Removed, all config consts have been
moved to the classes using them and KeyFile convenience methods
subsumed by ConfigFile.
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