Add boutton on "Activity Price"

Hi,

Activity Price Module is designed to define price of all activities of a given type on a given project.

What you expect to do is to define an Activity Price for a given activity.
You can do this just by adding field idActivity :
- in the database table activity, with format int(12).
- in /model/Activity.php in the list of fields for the class.
That's all.

But the billing module will not take this new field into account.
You'll have to update many other files, and it is much more complex.