Bitcoin-Qt: fixes for using display unit from options

- extend PaymentServer with setOptionsModel() and rework initNetManager()
  to make use of that
- fix all other places in the code to use display unit from options and no
  hard-coded unit

Conflicts:
	src/qt/bitcoin.cpp
	src/qt/bitcoingui.cpp
	src/qt/paymentserver.cpp
	src/qt/paymentserver.h
	src/qt/sendcoinsdialog.cpp
	src/qt/test/paymentservertests.cpp
	src/qt/transactiondesc.cpp