Minimal architectural changes necessary to support multiple wallets in bitcoin-qt

- This commit is a minimal restructuring necessary to support multiple wallets in the UI. Please see multiwallet-qt.txt for details.

src/qt/bitcoin.cpp

@@ -232,7 +232,8 @@ int main(int argc, char *argv[])
                 WalletModel walletModel(pwalletMain, &optionsModel);
 
                 window.setClientModel(&clientModel);
-                window.setWalletModel(&walletModel);
+                window.addWallet("~Default", &walletModel);
+                window.setCurrentWallet("~Default");
 
                 // If -min option passed, start window minimized.
                 if(GetBoolArg("-min"))
@@ -253,7 +254,7 @@ int main(int argc, char *argv[])
 
                 window.hide();
                 window.setClientModel(0);
-                window.setWalletModel(0);
+                window.removeAllWallets();
                 guiref = 0;
             }
             // Shutdown the core and its threads, but don't exit Bitcoin-Qt here