Simplify sending files from systray icon

When using the KDE Connecct appindicator/systray icon (not the Plasmoid), sending files to a device requires some extra clicks due to the intermediate dialog that we show.

This dialog lets you send URLs in addition to files, but given sending files is the most common use case and URLs can already be shared using the command line tool, the browser extension or just via the clipboard sync, we want to make this option open the file picker directly without the intermediate dialog.

Unlike in the current implementation, this should allow selecting and sending more than one file at a time.

Showing the file picker directly is also the behavior we have in the Plasmoid and the app.