Translate support in content edit

*Title: Translate support in content edit

*Incentive:
Make it easier to perform translations of content from one
language to another. Instead of using a separate edit screen
the same as normal editing should be used.
This allows the user to have all the controls he would normally
have.

*Documentation:
A new parameter was added to content/edit called FromLanguage which
when specified will show attribute data from this language beside/above
the attribute that is being edited.

For instance to translate from eng-GB and into nor-NO
content/edit/10/1/nor-NO/eng-GB

Also new template variables has been available to edit.tpl:
- from_language - The language entered in the url or false if none
- from_content_attributes - An array with attributes taken from FromLanguage or false if no such language
- is_translating_content - Set to true if the edit page is translating from another language, false otherwise