Dynamic mission objectives

[nkrisztian89]

• add new fields to mission events: active (default: true)
• add a new Action type for activating / deactivating events (params are event name and active state)
• inactive events do not check their trigger
• inactive events do not show up as mission objectives
• when a new mission objective is activated, a HUD message should be posted automatically, announcing the new objective (with a custom sound effect - #271), and the objective change animation (#279) should trigger
• add a new field to mission events: hiddenObjective (default: false) - when true, the associated mission objective should not show up in the mission briefing (for objectives that are there to activate the main, final objective)