refactor: improve invoice download dialog UI/UX

- Modified 'Dialogs.css' to replace the button-based UI with a dropdown select for invoice download
options.
- Refactored the relevant methods in 'Dialogs.js' accordingly, removing button event listeners and
implementing new logic for select-based UI.
- Replaced 'QUIPopup' with 'QUIConfirm' to incorporate an 'OK' button for confirmation.
- Changed hardcoded button options to select options in the DOM structure.
- Altered event listeners in 'Dialogs.js' to suit the new 'select' element and handle submission of
invoice download requests differently.
- Preserved the behavior of injecting an iframe for actual downloads, with necessary changes in
parameters and references now pointing to the select value instead of button value.

Related: quiqqer/invoice#157