Reject dust amounts during validation

Replaces the validation check for "amount == 0" with an isDust check,
so very small output amounts are caught before the wallet
is unlocked, a transaction is created, etc.

src/qt/guiutil.h

@@ -36,6 +36,9 @@ namespace GUIUtil
     bool parseBitcoinURI(const QUrl &uri, SendCoinsRecipient *out);
     bool parseBitcoinURI(QString uri, SendCoinsRecipient *out);
 
+    // Returns true if given address+amount meets "dust" definition
+    bool isDust(const QString& address, qint64 amount);
+
     // HTML escaping for rich text controls
     QString HtmlEscape(const QString& str, bool fMultiLine=false);
     QString HtmlEscape(const std::string& str, bool fMultiLine=false);