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/main.cpp

@@ -2617,9 +2617,13 @@ string GetWarnings(string strFor)
     int nPriority = 0;
     string strStatusBar;
     string strRPC;
+
     if (GetBoolArg("-testsafemode"))
         strRPC = "test";
 
+    if (!CLIENT_VERSION_IS_RELEASE)
+        strStatusBar = _("This is a pre-release test build - use at your own risk - do not use for mining or merchant applications");
+
     // Misc warnings like out of disk space and clock is wrong
     if (strMiscWarning != "")
     {