Add `progress` field to sets

[bennetrr]

Currently, sets only have the finished property that is set automatically when all parts are complete.

Add a progress field with multiple values:

• todo: Default value
• in-progress`: Set automatically if a part is added
• finished: Set automatically if all parts are complete, if set by user and there are parts missing, the missing parts are displayed in the part order list (Add part order list #53)
• selling: When the set was added to Kleinanzeigen or another private sale platform
• sold/stored: When the set was sold or stored in the Lego shelve