feat: Allow sending files by copy-pasting them from file managers

• Gtk.Clipboard.wait_for_uris() and Gtk.Clipboard.wait_is_uris_available() need to be typed somewhere
• Is there a graceful way to prevent TextView from insolently pasting text despite the fact we're handling paste-clipboard ourselves?
• Separate commit for send-file action refactoring?
• Better commit message wording