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/clientmodel.h

@@ -42,6 +42,7 @@ public:
 
     QString formatFullVersion() const;
     QString formatBuildDate() const;
+    bool isReleaseVersion() const;
     QString clientName() const;
     QString formatClientStartupTime() const;
 
@@ -60,6 +61,7 @@ private:
 signals:
     void numConnectionsChanged(int count);
     void numBlocksChanged(int count, int countOfPeers);
+    void alertsChanged(const QString &warnings);
 
     //! Asynchronous error notification
     void error(const QString &title, const QString &message, bool modal);