refactor invoice editing form code

[jzarate]

refactor most of the code used in the forms involved in invoice editing.

[jzarate]

r162

[jzarate]

r173 mayor refactoring. also partials for invoiceitem and tax

[jzarate]

r208 variable use simplified and recurrent line removed from actions.class.php

[jzarate]

r226 removing invoice items / taxes is no longer an ajax call. no change will be made untill the user press "save"

[carlos]

I figure you know it but, if not, you have to know that it seems that totals are not being correctly calculated in invoice editing form.