Add event check when deciding if a mission can be launched

[RealityMachina]

in UISquadSelect, it runs some logic in CanLaunchMission to decide if a player can launch a mission. An custom event check for mods to run additional logic there (ie if a specific class of unit is needed) would be appreciated.

[Iridar]

Having an event wouldn't hurt, but as it is it's possible to achieve this via SitRep or custom Mission State.

[Iridar]

Note to self: check if it's possible to make the event pass a localized messaged as to why the mission can't be launched. If not, the whole thing is probably not worth bothering with.