Change Manifest Metadata Fields with Predefined Set of Valid Values from Free Text to Drop-down

[schwemmer]

Check all manifest level metadata fields in the spec and the ones that only accept certain valid values should be changed from free text to drop-down with valid values.

Example:
The drop-down menu should have the following options:

• left-to-right
• right-to-left
• top-to-bottom
• bottom-to-top