[Qt] additional small fixes for #3099 (new receive flow)

- remove 2 unneeded windowTitle attributes, which bloat our translations
- cleanup some unneeded .cpp/.h includes and class usages
- use a more generic string for clearing sendcoinsdialog and
  requestpaymentdialog
- edit 2 strings in BitcoinGUI and replace "edit" with "show" as this
  seems more clear in the context where it is used

src/qt/forms/receiverequestdialog.ui

@@ -10,9 +10,6 @@
     <height>597</height>
    </rect>
   </property>
-  <property name="windowTitle">
-   <string>Request coins</string>
-  </property>
   <layout class="QVBoxLayout" name="verticalLayout_3">
    <item>
     <widget class="QRImageWidget" name="lblQRCode">