Show syncing warning when sending tx

src/confirm.ui

@@ -146,6 +146,19 @@
      </property>
     </spacer>
    </item>
+   <item>
+    <widget class="QLabel" name="syncingWarning">
+     <property name="styleSheet">
+      <string notr="true">color: red;</string>
+     </property>
+     <property name="text">
+      <string>You are sending a transaction while your node is still syncing. This may not work.</string>
+     </property>
+     <property name="wordWrap">
+      <bool>true</bool>
+     </property>
+    </widget>
+   </item>
    <item>
     <widget class="QLabel" name="warningLabel">
      <property name="styleSheet">

src/sendtab.cpp

@@ -505,6 +505,9 @@ bool MainWindow::confirmTx(Tx tx) {
         }
     }
 
+    // Syncing warning
+    confirm.syncingWarning->setVisible(Settings::getInstance()->isSyncing());
+
     // And FromAddress in the confirm dialog 
     confirm.sendFrom->setText(fnSplitAddressForWrap(tx.fromAddr));