Add the `__dry_update` command

The user can call __dry_update to only refresh the list of available karas from Kurisu without downloading them.

If a kara has changed metadata or is updated on Kurisu, the change won't be visible after a dry update. We need another system or change the database scheme to take into account this case.

Partially solves #102 (closed)

added Feature label

assigned to @martin2018

merged