startcenter: react on mouse down instead

There is a good reason for that: File lists of
some file pickers handle double click event on
mouse down. So if a start center thumbnail is
exactly behind the file on which the user double-
clicked, it will receive the mouse up event, and
will also open.

Change-Id: I9fc9ceff0874d0486d5b568c9b35143c8d8aabf8
Fix indentation

Change-Id: I7ac9792da93a1b5231ddac5398823667b28adf92