[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/sendcoinsdialog.ui

@@ -84,7 +84,7 @@
         </sizepolicy>
        </property>
        <property name="toolTip">
-        <string>Remove all transaction fields</string>
+        <string>Clear all fields of the form.</string>
        </property>
        <property name="text">
         <string>Clear &amp;All</string>