Fix 'undefined variable' notice when submitting field settings form with 'php code for default value.

#336174 Move actual field access check in content.module, with content_permission.module providing one permission-based implementation.

Diff integration: refactored around a new hook_content_diff_values() to save contrib field modules the
burden of implementing hook_diff. For most field types, the default content_field_content_diff_values()
should be enough, though.

+ abstract a few element validators

#331995 Fix 'invalid argument for foreach' warning on nodes with inexistent type. (#327715 followup - content_types() *always* returns a non-array, so testing "if ($type =  content_types($foo))" is useless

content_field_instance_read($include_inactive == TRUE) was generating a broken query - it so happened that the query still returned the expected results :-)

Adding TODO to content_generate code. It currently works, but needs to be reworked sometime post-release.

#311912 Views integration: The many_to_one filters for fields with 'allowed values' gain should not replace the regular 'starts with'/'greater than' filters.

#329037 Fix small bug in content_generate function method of calling fields that handle their own multiple values.

- a few relationship-safe fixes after merlinofchaos had a look a our code :-)
- a little refactoring to make content_handler_field / content_handler_field_multiple articulation clearer.

+ remove dead code in content.panels.inc

#320632 Content Copy: Make fields/groups checked for export by default + display the list in an overview table.