qt: keep a list of requested payments

Keep a list of requested payments in the Receive tab so that a user can
recall previously created requests after closing their windows.

Currently this list is not stored between bitcoin-qt sessions. This can
be implemented later, but it is not clear where it should be stored as
I don't think it belongs in the wallet (maybe in QSettings?)

src/qt/forms/receiverequestdialog.ui

@@ -83,13 +83,6 @@
        </property>
       </widget>
      </item>
-     <item>
-      <widget class="QPushButton" name="btnCopyImage">
-       <property name="text">
-        <string>&amp;Copy Image</string>
-       </property>
-      </widget>
-     </item>
      <item>
       <widget class="QPushButton" name="btnSaveAs">
        <property name="text">