Replace the term "Force" by "Velocity"

[D8H]

I think some actions use the term "Force" instead of "Velocity". If this is really not the right term, I see 2 reasons that can motivate a renaming:

• it could mislead children to what a force really is
• teachers could choose not use GDevelop worrying about the previous point

Some of these expression descriptions also use "Average" instead of "Sum".