Move SearchFolder management into the client

Rename the search folder again to App.SearchFolder, move its id class
into it as an inner class. Remove search folder from the engine so the
application can manage it and it's policy in the future. Also remove
the outbox from the accout's list of local folders, so that code can
be removed altogether.

src/engine/meson.build

@@ -37,7 +37,6 @@ geary_engine_vala_sources = files(
   'api/geary-problem-report.vala',
   'api/geary-progress-monitor.vala',
   'api/geary-revokable.vala',
-  'api/geary-search-folder.vala',
   'api/geary-search-query.vala',
   'api/geary-service-information.vala',
   'api/geary-service-provider.vala',
@@ -47,6 +46,7 @@ geary_engine_vala_sources = files(
   'app/app-conversation-monitor.vala',
   'app/app-draft-manager.vala',
   'app/app-email-store.vala',
+  'app/app-search-folder.vala',
 
   'app/conversation-monitor/app-append-operation.vala',
   'app/conversation-monitor/app-conversation-operation-queue.vala',
@@ -197,7 +197,6 @@ geary_engine_vala_sources = files(
   'imap-engine/gmail/imap-engine-gmail-all-mail-folder.vala',
   'imap-engine/gmail/imap-engine-gmail-drafts-folder.vala',
   'imap-engine/gmail/imap-engine-gmail-folder.vala',
-  'imap-engine/gmail/imap-engine-gmail-search-folder.vala',
   'imap-engine/gmail/imap-engine-gmail-spam-trash-folder.vala',
   'imap-engine/other/imap-engine-other-account.vala',
   'imap-engine/other/imap-engine-other-folder.vala',
@@ -270,9 +269,6 @@ geary_engine_vala_sources = files(
   'rfc822/rfc822-part.vala',
   'rfc822/rfc822-utils.vala',
 
-  'search/search-email-identifier.vala',
-  'search/search-folder-impl.vala',
-
   'smtp/smtp-authenticator.vala',
   'smtp/smtp-capabilities.vala',
   'smtp/smtp-client-connection.vala',