Show warning when using prerelease version

Implements #1948

- Add macro `CLIENT_VERSION_IS_RELEASE` to clientversion.h
- When running a prerelease (the above macro is `false`):
  - In UI, show an orange warning bar at the top. This will be used for other
    warnings (and alerts) as well, instead of the status bar.
  - For `bitcoind`, show the warning in the "errors" field in `getinfo`
    response.

src/qt/overviewpage.h

@@ -10,6 +10,7 @@ QT_END_NAMESPACE
 namespace Ui {
     class OverviewPage;
 }
+class ClientModel;
 class WalletModel;
 class TxViewDelegate;
 class TransactionFilterProxy;
@@ -23,7 +24,8 @@ public:
     explicit OverviewPage(QWidget *parent = 0);
     ~OverviewPage();
 
-    void setModel(WalletModel *model);
+    void setClientModel(ClientModel *clientModel);
+    void setWalletModel(WalletModel *walletModel);
     void showOutOfSyncWarning(bool fShow);
 
 public slots:
@@ -35,7 +37,8 @@ signals:
 
 private:
     Ui::OverviewPage *ui;
-    WalletModel *model;
+    ClientModel *clientModel;
+    WalletModel *walletModel;
     qint64 currentBalance;
     qint64 currentUnconfirmedBalance;
     qint64 currentImmatureBalance;
@@ -46,6 +49,7 @@ private:
 private slots:
     void updateDisplayUnit();
     void handleTransactionClicked(const QModelIndex &index);
+    void updateAlerts(const QString &warnings);
 };
 
 #endif // OVERVIEWPAGE_H