Building jobs

Add: built mortars have Fire_Mortar job (doesn't do anything yet)
Add: logic so units won't take jobs on occupied tiles
Add: building jobs destroyed if building destroyed
Refactor: job times now in JobTimes class instead of magic numbers
Fix: give mortar buildings z-dimension
Sentries & player mortars

Add: player mortar fire
Add: sentry job
Remove: cancel job button
Sentry placement fix

Fix: position restrictions for sentry