Bitcoin-Qt: never display own block count > estimated block count

- some users reported it as weird, that the estimated block count could be
  lower than our own nodes block number (which is indeed true and not good)
- this pull adds a new default behaviour, which displays our own block
  number as estimated block number, if own >= est. block count
- the pull raises space for nodes block counts in cPeerBlockCounts to 8 to
  be more accurate
- also removes a reduntant setNumBlocks() call in RPCConsole and moves
  initialisation of numBlocksAtStartup in ClientModel, where it belongs
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
Change progress bar from block-based to time-based

This is less confusing to most people, and doesn't rely on estimates
of the total number of blocks received from other nodes.

Conflicts:
	src/qt/bitcoingui.cpp
Remove NumBlocksOfPeers

Generally useless information. Only updates on connect time, not after
that. Peers can easily lie and the median filter is not effective in
preventing that.

In the past it was used for progress display in the GUI but
`CheckPoints::guessVerificationProgress` provides a better way that is now used.
It was too easy to mislead it. Peers do lie about it in practice, see issue #4065.

From the RPC, `getpeerinfo` gives the peer raw values, which are more
useful.

Conflicts:
	src/main.cpp
	src/main.h
	src/qt/bitcoingui.h
	src/qt/clientmodel.cpp
	src/qt/clientmodel.h
	src/qt/forms/rpcconsole.ui
	src/qt/rpcconsole.cpp
qt: Show weeks as well as years behind for long timespans

Closes #3811.

Conflicts:
	src/qt/locale/bitcoin_en.ts