Extend functionality to all element types

[matfish2]

#3

Currently supports only Entry.

Suggested behavior:

1. Add settings page where user can select elements (Entry, Category, Tag, User, Asset, Product, Variant) to which he wishes to add metadata.
2. Indicate existing metadata columns (rename to emMetadata to avoid collision), and allow removal
3. Allow "Other" option, where user can add a fully qualified path to a custom element + active record.
   a. Validate paths
   b. Add metadata column to table associated with active record
   c. Register element and query behaviors on respective classes.

[matfish2]

Added
Craft 3: >=1.1.1
Craft 4: >=3.0.0