[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