FilePicker: add an option to specify also the file name

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.

Locate the FilePicker Vue component and its buttons callback, then read how allowPickDirectory currently passes Folder items. Confirm the existing behavior when no fileName option is provided and decide how the new filename value can be exposed without breaking that callback. Done means the optional editable filename input and callback behavior are specified and covered by the component's existing tests.