[downloads] Check whether the filename exists with ".part" before using it
[downloads] Check whether the filename exists with ".part" before using it

In CalculateFilename, when checking whether we should generate a new
filename it now also checks whether the old filename exists with
".part" appended. This avoids clobbering an existing download.

clutter-project/clutter-mozembed,clutter-project/clutter-mozembed,clutter-project/clutter-mozembed