Support unlocking VeraCrypt hidden volumes via udisks

During the UX sprint we decided that we must support unlocking hidden volumes, because it is an often used feature (65% of survey participants).

This is already supported by cryptsetup, so we will have to patch libblockdev and udisks for this.

Original created by @segfault on 15045 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information