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.