Add some hooks into pac part logic

Adds three hooks that can be used to write pac parts with specialized
logic:

"pac_PART:SetTable" is run for each key/value pair a part is getting
from the table passed to SetTable. This most notably occurs when the pac
editor loads parts from file

"pac_model:SetModel" is run when a model part is about to set its model

"pac_pace.SaveParts" is run when the editor has constructed a table
representing the parts and is about to save it to file

(the vfs part uses these hooks)