Fix default-constructed QFileSystemEntry

Fix default-constructed QFileSystemEntry

Member variables for lastSeparator, first and lastDotInFileName are now
initialized to -1 (non-existing), where the previous value of zero would
mean a separator/dot at that position and resulted in path() returning
'/', instead of '.'.

Tests were expanded for better coverage of empty state and
default-constructed instances.

Change-Id: Ie27547886b52224d38b5be0b4f920c9927fd440f
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>