update CClientUIInterface and remove orphan Wx stuff

- fix ThreadSafeMessageBox always displays error icon
- allow to specify MSG_ERROR / MSG_WARNING or MSG_INFORMATION without a
  custom caption / title
- allow to specify CClientUIInterface::ICON_ERROR / ICON_WARNING and
  ICON_INFORMATION (which is default) as message box icon
- remove CClientUIInterface::OK from ThreadSafeMessageBox-calls, as
  the OK button will be set as default, if none is specified
- prepend "Bitcoin - " to used captions
- rename BitcoinGUI::error() -> BitcoinGUI::message() and add function
  documentation
- change all style parameters and enum flags to unsigned
- update code to use that new API

- update Client- and WalletModel to use new BitcoinGUI::message() and
  rename the classes error() method into message()
- include the possibility to supply the wanted icon for messages from
  Client- and WalletModel via "style" parameter

Conflicts:
	src/bitcoinrpc.cpp
	src/init.cpp
	src/main.cpp
	src/noui.cpp
	src/qt/bitcoingui.cpp
	src/qt/walletmodel.h
	src/util.cpp
fix some double-spaces in strings

- remove some unneeded stuff in sendcoinsentry.ui
- harmonize some "Error:"-messages

Conflicts:
	src/init.cpp
	src/qt/bitcoingui.cpp
	src/qt/forms/sendcoinsentry.ui
	src/rpcwallet.cpp
	src/wallet.cpp
qt: Log messages with type>QtDebugMsg as non-debug

More important messages should end up in the log no matter
if -debug=qt is set.

Conflicts:
	src/qt/bitcoin.cpp
Fix the Qt5 build after d95ba75

Sorry, my own fault this time.