Allow user to display firstname before lastname

[Yajo]

For many users this is a better way of seeing the contact list.

[Alexis de Lattre (alexis-via)]

I am also affected by this issue. I would like to have the firstname before the lastname.

I see 2 solutions :
- make it a configurable option
- make it inheritable

Currently, in the code of partner_firstname, the "name" field is converted to a fields.function which calls _compute_name_custom and all the code is inside the function _compute_name_custom, so we can't inherit it. I propose to move the code that is inside the loop of the function _compute_name_custom to a dedicated function that can be inherited.

I'll to the MP tonight if you agree.