It doesn't contain any useful information.

To ensure that the result of an Activate/Deactivate call is either the
expected state or an error (which nudges users to send a bug report).

Changes of the IsActive and IsEnabled properties were signaled with the
same PropertiesChanged signal, causing the UI to briefly display an
"inconsisteng state" error. We now signal changes of IsEnabled, IsActive
and HasData all with the same signal.

Move the sync call after the mount call to ensure that we still raise a
MountException first which is what the tests check for.

Activation errors are now encoded by IsEnabled being true and IsActive
being false.

The methods should be idempotent now, so instead of refreshing the state
at the beginning (which is quite costly) and raising an error, just do the
activation/deactivation again.

The semantic is similar to systemd units:
* IsEnabled means the feature is configured to be activated on boot
* IsActive means it's currently active

This required:
* Adding a translatable name to the features in the backend
* Making the greeter able to translate to the currently selected language
  via a global gettext function which now lives in config.gettext. This
  was required because before, all translations were really only done by
  TranslatableWindow which translates all its labels automatically - but
  it translates the full label string, which doesn't work when the label
  is composed of multiple strings which must be translated separately,
  like the feature names we want to include in the message (because we
  can't make translators translate the complete error message for all
  possible combinations of features).

And use the subtitle of the HdyActionRow.

And remove the GtkExpander.

It still doesn't look good, the space between the first and second row
is too big but we can't change that unless we stop using HdyActionRow.

Closes #19368

It was confusing for users that the error message said "Failed to
activate the Persistent Storage" even though most of the Persistent
Storage features worked as expected.

For consistency with GNOME (e.g. GNOME Settings) and other Tails apps
(e.g. TCA) we also use the unicode character … instead of three period
characters in button names.

Adding an expander to the row above makes it more clear that the row
with the Delete Data button belongs to the row above.

refs #8447

Closes #19291

Clicking the button when the feature was inactive caused the app to
crash.

Else the tps backend fails to (re-)start when there is inconsistent state.

... without running the on-deactivated hooks in between. That might
happen when a feature that's deactivated and activated again is in
inconsistent state.

The only hooks which did not behave as expected when run twice were the
add-gnome-bookmarks hooks, which added the bookmarks twice. They now
only add the bookmarks when the bookmarks don't exist yet.

This reverts commit 136a7030, which was
a workaround for one case where the bad handling of inconsistent state in
the refresh_is_active method lead to a bug.

Commit "tps: Fix activation of feature with inconsistent state" is a
more general solution, which makes the workaround unnecessary:
refresh_is_active will not set IsActive for any features whose bindings
are not active, so it's a no-op after a call to Unlock (unless a symlink
feature is enabled in the config file and there are still valid
symlinks, in which case it's correct that the feature is treated as
active).

We used to set Feature.IsActive property only based on whether the
feature is in the config file. That meant that we set it to True even if
the mounts were not all active, which led to errors when activating a
feature which was in inconsistent state (the feature being in the config
file but not all mounts being active).

This should also avoid (potential) issues where after a method call that
failed or was cancelled, a feature would have IsActive set to True (because
it's in the config file) but the mounts not being active, causing the
feature to be displayed as active in the UI but the user's data not actually
being stored. We now set IsActive to False in that case.

Closes #19376

Persistent Storage design docs

Closes #19223

See merge request tails/tails!1035