[Task Manager] Allow non-file "Recent Documents"

This allows recent documents to include folders (e.g. visited places
in Dolphin) as well as other protocols, such as "vnc://foo" for
recently connected remote desktops.

Moreover, it adjusts the "Recent Files" heading to be more precise:

* "Recent Downloads" when all files are in the Downloads folder, e.g.
  for a browser with plasma-browser-integration
* "Recent Places" when all items are files, e.g. for a file manager
* "Recent Connections" when all items are remote URLs without file name,
  e.g. remote desktop or server URLs