Update UI through async calls MainFrameRepaint and AddressBookRepaint instead of a timer.

- Overall, this is better design
- This fixes problems with the address book UI not updating when the address book is changed through RPC
- Move Statusbar change detection responsibility to ClientModel

src/qt/clientmodel.h

@@ -43,6 +43,7 @@ private:
 
     int cachedNumConnections;
     int cachedNumBlocks;
+    QString cachedStatusBar;
 
     int numBlocksAtStartup;