Suggest filesystem label when formatting

The format dialog required the used to specify the filesystem label in order to proceed.

Lift this restriction and provide a default filesystem label based on a heuristic device identifier and the partition size.

Original discussion in: !5 (closed)

I tested the suggested label for internal drives, with LUKS (with and without a partition name), and on removable drives.

@oholy Could you review this? Maybe you have a tip or can test it as well. Thanks!