Enable simple changes to custom data-types

Certain changes to custom data fields can be safely performed with little or no change to the underlying tables. For example, an alphanumeric "Select" can be converted to an alphanumeric "Radio" without any data changes, and an alphanumeric "Radio" can be converted to an alphanumeric "CheckBox" with a small data change (i.e. adding delimiter characters).

The attached patch implements some basic "change-type" functionality. It's implemented in the style of CRM_Custom_Form_MoveField – i.e. it creates a hidden form which can be accessed by manually entering a URL like:

https://example.com/civicrm/admin/custom/group/field/changetype?reset=1&fid=117

The page can be exposed more publicly when the feature has been more thoroughly tested and implemented.