[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] 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
Bugfix: configure: Allow user-provided CXXFLAGS to override default optimisation, debug, and warning flags
Bugfix: Undefine _FORTIFY_SOURCE before redefining it, to avoid warnings on compilers that define it by default
Cleanup LevelDB library dependencies
configure: Internal changes to make building bitcoind and bitcoin-cli optional
Bugfix: Correct captitalisation of Qt
configure: Change --with[out]-qt to --with[out]-gui and add --with[out]-cli and --with[out]-daemon to support more build configurations
configure: Minor grammatical changes to be more clear
Bugfix: Use BOOST_CPPFLAGS when testing for boost sleep vs sleep_for
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)
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`.