Fix image title and alt and file display settings, which were not getting picked up in the migration.

Issue #1117028 Add messages to the textfield changes. Make an assumption that a text with no maxlength that uses a textfield widget was intended to be a varchar field with a textfield widget. Users can see from the message what will happen so they will not migrate the field if that is not desired.

Ensure that text filter formats get set to an allowed value during the migration, borrowing code from node_update_7006().

Issue #1039466 We need to switch the file module for file and image fields even if they use a custom widget or they will not show up as fields that can be migrated. Best we can do is display anything that has a 'imagefield' or 'filefield' prefix.

Issue #1093844 by dww, Fix PHP warnings from content_migrate.filefield.inc on undefined legacy settings.

#781088 Reorganize Content Migrate files for efficiency, and also to make it easier to use the individual module code as examples on how to implement the hooks.