FilePicker: add an option to specify also the file name

[madbob]

FilePicker misses an option to replicate a "Save As" functionality, in which the user can specify not only a path but also a (new) file name.

Probably this can be done with a new prop for the Vue component, fileName. If it is false (the default), just do nothing more than now. If it is a string, display a new editable text input field inited with the specified string.

It is debatable now to pass this new information to the buttons callback. Existing implementations with allowPickDirectory = true expect just a Folder item, and of course this cannot be broken; perhaps the callback can be called as callback(nodes, filename) (using the value of the new text input) when fileName != false.

[susnux]

You can already do so, just put in you custom button handler.
E.g. specify a button with text "save as" and the handler of that button opens a new input dialog for the filename.

[madbob]

A button to just save, a button to specify a filename, a button to define tags (cfr #2510)... The "Save attachment to Files" dialog in Mail is going to be crowded...
Anyway, that's fine.

I bet sooner or later more Nextcloud apps importing documents to Files will need this same functionality. That day I hope not to see four different implementation of "Filename" custom buttons scattered across different apps, but I guess it is premature right now.