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.cpp

@@ -88,9 +88,7 @@ void ClientModel::updateAlert(const QString &hash, int status)
         }
     }
 
-    // Emit a numBlocksChanged when the status message changes,
-    // so that the view recomputes and updates the status bar.
-    emit numBlocksChanged(getNumBlocks(), getNumBlocksOfPeers());
+    emit alertsChanged(getStatusBarWarnings());
 }
 
 bool ClientModel::isTestNet() const
@@ -133,6 +131,11 @@ QString ClientModel::formatBuildDate() const
     return QString::fromStdString(CLIENT_DATE);
 }
 
+bool ClientModel::isReleaseVersion() const
+{
+    return CLIENT_VERSION_IS_RELEASE;
+}
+
 QString ClientModel::clientName() const
 {
     return QString::fromStdString(CLIENT_NAME);