Push all ImapDB path management down into to ImapDb.Account.

This stops generating account storage paths, and in particular the
location of attachment files, in a number of different places. Instead,
we determine the paths once, in ImapDb.Account and pass them around as
needed. This will help making the ImapDB classes unit-testable, and
in particular ImapDb.Folder by passing an instance of Db.Database,
rather than ImapDb.Database.

test/engine/imap-db/imap-db-database-test.vala

@@ -20,8 +20,9 @@ class Geary.ImapDB.DatabaseTest : TestCase {
         );
 
         Database db = new Database(
-            tmp_dir,
+            tmp_dir.get_child("test.db"),
             GLib.File.new_for_path(_SOURCE_ROOT_DIR).get_child("sql"),
+            tmp_dir.get_child("attachments"),
             new Geary.SimpleProgressMonitor(Geary.ProgressType.DB_UPGRADE),
             new Geary.SimpleProgressMonitor(Geary.ProgressType.DB_VACUUM),
             "test@example.com"