add DEFAULT_TRANSACTION_FEE constant in wallet

- as this is a shared Core/GUI setting, this makes it easier to keep them
  in sync (also no new includes are needed)
rpc: add `getblockchaininfo` and `getnetworkinfo`

Adds two new info query commands that take over information from
hodge-podge `getinfo`.

Also some new information is added:
- `getblockchaininfo`
  - `chain`: (string) current chain (main, testnet3, regtest)
  - `verificationprogress: (numeric) estimated verification progress
  - `chainwork`
- `getnetworkinfo`
  - `localaddresses`: (array) local addresses, from mapLocalHost (fixes #1734)
qt: periodic translations update
Merge pull request #4127

d387b8e rpc: add `getblockchaininfo` and `getnetworkinfo` (Wladimir J. van der Laan)
build: fix make clean on OSX

Rebased-From: 74df0f5
Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
ui: Check for !pixmap() before trying to export QR code

Adds null pointer checks as well as prevents the Save/Copy context
menu from appearing at all if no image is shown.

Fixes issue #4140
Add missing cs_main lock to VerifyDB

Fixes issue #4139.
[Qt] fix ugly typo in bitcoin.cpp

Closes #4145.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 1e4b083
Merge pull request #4141

9d558e1 ui: Check for !pixmap() before trying to export QR code (Wladimir J. van der Laan)
Merge pull request #4131

3e1cf9b add DEFAULT_TRANSACTION_FEE constant in wallet (Philip Kaufmann)
Typo Fix In decoderawtransaction Help: s/txid/hex/

Help text appears to have been copy/pasted from getrawtransaction,
so it erroneously asked for a txid where rawtransaction hex should appear.

Remove lines which were copy/pasted from getrawtransaction but which
aren't displayed by decoderawtransaction.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 5cc0133 80c521e
Github-Pull: #4106
qt: fix compile issue in Qt GUI

This was introduced in 3e1cf9b. Needs a cast to qint64.
Split up CheckBlock in a block and header version
Split AcceptBlockHeader from AcceptBlock.

Also modify some connection logic to deal with non-full blocks in the index.
Get rid of the static chainMostWork (optimization)
Allow ActivateBestChain to release its lock on cs_main
Move all post-chaintip-change notifications to ActivateBestChain
Push cs_mains down in ProcessBlock