When the user opens (from Dolphin) an archive whose plugin is not available,
the feedback is currently very annoying:

• a KMessageBox pops up with the error message
• the user closes the message box
• an empty (and useless) Ark window shows up

This patch replaces the KMessageBoxes with KMessageWidgets, so that the 3 steps
above are merged into 1.

Possible alternative solution: display only the KMessageBox, then quit Ark
(preventing the MainWindow from showing up). But I'm not sure if this is
actually better.

This patch also move the input validation code from Part::openFile() to a new
Part::isLocalFileValid() method.