Commit
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.

Conflicts:
	doc/release-process.txt
	src/qt/bitcoingui.cpp
	src/qt/clientmodel.h
	src/qt/forms/overviewpage.ui
	src/qt/overviewpage.cpp

Conflicts:
	src/qt/bitcoingui.cpp
	src/qt/clientmodel.h