Fire hook_field_load and hook_field_attach_load for fields when aggregation is used.

Problem/Motivation

If I use entity fields as output and enable grouping in some occasions I get notices about undefined indexes and broken field output.
Some of these notices can be avoided by adding extra grouping fields in the Aggregation settings of a field.
E.g on a text field you can include "Format" to ensure this field value item is available for later use.
Unfortunately there are some "hidden" field value items e.g. "safe_value" on text fields. Those "hidden" field value items currently aren't addressed by views_handler_field_field.

Those "hidden" field value items are handled by hook_field_load and hook_field_attach_load.
These hooks are fired whenever an entity is loaded.
If aggregation is used views_handler_field_field modifies the field item values, to inject the views specific field output, of the returned entities.
Doing so the field item values are overwritten completely - and thus everything that's not explicitely declared in the Aggregation settings is lost.
The in code documentation says:

// Note: We would copy original values here, but it can cause problems.
// For example, text fields store cached filtered values as
// 'safe_value' which doesn't appear anywhere in the field definition
// so we can't affect it. Other side effects could happen similarly.

Proposed resolution

As disscues with dereine via IRC we should fire hook_field_load and hook_field_attach_load once the field item values are overwritten, to maintain the "hidden" field item values.
dereine already proposed a solution: http://paste.pocoo.org/show/540775/
The attached patch tries to take care about the // @todo: This hook should actually use the multiple execution.
To do so quite a bit of the post processing in views_handler_field_field was changed, including a function signature.

Remaining tasks

• Definitely needs a good review and heavy testing to ensure nothing bad is introduced.
  Latest patch #1
• Define an upgrade strategy - the fact that Entity API breaks with this patch shows how tricky this change is. See #1
• Define what has to happen with multi-value fields when aggregation is used. See #1.

User interface changes

None

API changes

• views_handler_field_field::set_items()
  Accepted parameters changed from ($values, $row_id) to ($entity_type, $entity, $display).
  Return is the same as before.
  In views itself this method is used just internally by views_handler_field_field.