Clean up search folder implementation

Move SearchFolder and search EmailIdentifier implementation out of
ImapDb and into its own package. Decouple both from ImapDB, and improve
the implementation, fixing a few inefficiencies. Merge search
FolderProperties into the SearchFoldern implementation as an inner
class.

Merge SearchTerm into ImapDB.SearchQuery as an inner class and move the
outer class's source down a level, since it was the only file left in
the imap-db/search dir.

test/engine/imap-engine/imap-engine-generic-account-test.vala

@@ -99,6 +99,21 @@ public class Geary.ImapEngine.GenericAccountTest : TestCase {
                 )
             )
         );
+        assert_non_null(
+            test_article.to_email_identifier(
+                new GLib.Variant(
+                    "(yr)",
+                    's',
+                    new GLib.Variant(
+                        "(vx)",
+                        new GLib.Variant(
+                            "(yr)", 'o', new GLib.Variant("(xx)", 1, 2)
+                        ),
+                        3
+                    )
+                )
+            )
+        );
     }
 
 }