Addition of MessageTable and MessageLocationTable toward fixing #3742.

Much of the API between the local and net stores had to be reworked for consistency as well as planning ahead for how messages will be retrieved and stored efficiently.  This work also attempts to keep in mind that other mail sources (POP, etc.) may be required in the future, and hopefully can be added without major rework.

src/engine/api/EmailProperties.vala

@@ -0,0 +1,9 @@
+/* Copyright 2011 Yorba Foundation
+ *
+ * This software is licensed under the GNU Lesser General Public License
+ * (version 2.1 or later).  See the COPYING file in this distribution. 
+ */
+
+public abstract class Geary.EmailProperties : Object {
+}
+