Support for more weapons

While the official mods shipping with OpenRA might not need it, more than two weapon slots could be useful for several other mods (including one I'm working on).

After Tertiary, I continued with Weapon4-9, as quaternary, quinary and so on are just asking for typos, much more difficult to remember and far less copy-paste-friendly.

NOTE: I did test this, and it seems to work fine, but I'm not a programmer, this was done by copy-pasting + trial & error. I'm particularly unsure about the MathMax part, I have no idea whether it really works as intended.

NOTE2: This feature would become more useful if someone could implement customizable per-turret graphics, so that Secondary and beyond can use different turret graphics instead of using copies of the first turret. That would allow a large ship to have a small machine gun turret, two large cannon turrets and a missile turret, for example. That's beyond my current abilities, though.