[Qt] let OptionsModel::getProxySettings() directly query proxy

- as a proxy set via GUI can be overridden via -proxy, directly query the
  core to get active proxy
- give a warning, if active proxy is not SOCKS5 (needs to be SOCKS5 for
  the Qt networking code to work)
- also remove an obsolete connect() call from optionsdialog.cpp and a
  reference to Bitcoin-Qt (now just GUI)
qt: allow `walletpassphrase` in debug console without -server

Currently it is only possible to use `walletpassphrase` to unlock the
wallet when bitcoin is started in server mode.

Almost everything that manipulates the wallet in the RPC console
needs the wallet to be unlocked and is thus unusable without -server.

This is pretty unintuitive to me, and I'm sure it's even more confusing
to users.

Solve this with a very minimal change: by making the GUI start a
dummy RPC thread just to handle timeouts.
[Qt] add utilitydialog.cpp/h and helpmessage class

- adds a nice and well formated dialog, which displays our -? help message
  (all options/paramaters)
- moves aboutdialog.cpp/h to the new utilitydialog
- move GUI shutdown window to utilitydialog
Fix importprivkey / rescan
Add a new checkpoint at block 279,000

From a cursory glance, it looks like block 279,000 meets the criteria for
a checkpoint block, and it's also what the bootstrap.dat torrent is at. This closes #3530.
Update linearize.py defaults to block 279,000
Merge pull request #3551

6873cfb Update linearize.py defaults to block 279,000 (Micha)
e7906d1 Add a new checkpoint at block 279,000 (Micha)
Update OS X Homebrew docs to include new build dependencies
wallet: add interface for storing generic data on destinations
[Qt] Permanently store requested payments in wallet
[Qt] Add sorting feature to the requested payments table
Merge pull request #3557

5d897f3 Update OS X Homebrew docs to include new build dependencies (Michael Ford)
Merge pull request #3242

2041365 Bugfix: Use BOOST_CPPFLAGS when testing for boost sleep vs sleep_for (Luke Dashjr)
e18e100 configure: Minor grammatical changes to be more clear (Luke Dashjr)
a9dbcf0 configure: Change --with[out]-qt to --with[out]-gui and add --with[out]-cli and --with[out]-daemon to support more build configurations (Luke Dashjr)
f472990 Bugfix: Correct captitalisation of Qt (Luke Dashjr)
f930341 configure: Internal changes to make building bitcoind and bitcoin-cli optional (Luke Dashjr)
4a290b5 Cleanup LevelDB library dependencies (Luke Dashjr)
9b4e03b Bugfix: Undefine _FORTIFY_SOURCE before redefining it, to avoid warnings on compilers that define it by default (Luke Dashjr)
123e4f8 Bugfix: configure: Allow user-provided CXXFLAGS to override default optimisation, debug, and warning flags (Luke Dashjr)
Merge pull request #3548

7b50bb2 [Qt] add utilitydialog.cpp/h and helpmessage class (Philip Kaufmann)
gitian: Windows 64 bit support

- Build a 64 bit version of all dependencies
- Show 32/64 bit version in "About..." for x86
- Export 64-bit .exes and installer from gitian build
build: Fix warning about space after trailing backslash
doc: Build with qt5 is possible again

This reverts commit 62568f9a1e98065bd51069649f23a337eab89f94.
Also add some documentation for `--with-gui`.
Merge pull request #3564

54d2b91 doc: Build with qt5 is possible again (Wladimir J. van der Laan)
Adapt Windows installer for 64 bit
build: Add bitcoin-cli.exe to installer