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

src/qt/transactiondesc.h

@@ -14,7 +14,7 @@ class TransactionDesc: public QObject
     Q_OBJECT
 
 public:
-    static QString toHTML(CWallet *wallet, CWalletTx &wtx);
+    static QString toHTML(CWallet *wallet, CWalletTx &wtx, int unit);
 
 private:
     TransactionDesc() {}