New preflights related to the stackup

[set-soft]

Three new preflights were added to help displaying PCB manufacture data in the PCB:

• update_pcb_characteristics: Once you insert the Board Characteristics (Place -> Add Board Characteristics) this preflight can be used to keep the information synced.
• update_stackup: Is similar but applies to the Stackup Table (Place -> Add Stackup Table)
• draw_stackup: Is the most powerful and can create a drawing for the stackup containing all the relevant information, i.e.