Improve save as dialog

Currently our save as dialog is rather simple and technical. It allows users to put files to a location by using an absolute path within Nextcloud.

For a better usability we could rather use the file picker from https://github.com/nextcloud-libraries/nextcloud-dialogs, however that currently does not allow you to cover the save as use case where you would provide a filename in it as well when you pick a folder location. We probably should extend the library as this may be useful also for the files app (e.g. when copying or moving a file to a new location where you can directly specify a new name) or other apps

Reference could be the usual operating system save as dialogs:

Start by reviewing the current save-as dialog and the file picker in the nextcloud-dialogs library. Clarify how a user should choose a folder and provide a filename, then compare the behavior with operating-system save-as dialogs. Done means the save-as flow supports selecting a location and specifying a filename without relying on an absolute path.