ui: Add "Copy raw transaction data" to transaction list context menu

Add a way to quickly copy transaction hex.

Primarily useful when manually submitting transactions,
e.g. `-walletbroadcast=0` is set.

src/qt/transactionview.h

@@ -93,6 +93,7 @@ private Q_SLOTS:
     void copyLabel();
     void copyAmount();
     void copyTxID();
+    void copyTxHex();
     void openThirdPartyTxUrl(QString url);
     void updateWatchOnlyColumn(bool fHaveWatchOnly);