Decrease Engineer, Medic and Mechanic priority

Fixes the issue that engineers would just run straight at the target while
mixed with a group and attack moved.

mods/ra/rules/infantry.yaml

@@ -182,6 +182,8 @@ E6:
 	-AutoTarget:
 	Voiced:
 		VoiceSet: EngineerVoice
+	Selectable:
+		Priority: 5
 
 SPY:
 	Inherits: ^Soldier
@@ -320,6 +322,8 @@ MEDI:
 		AttackSequence: heal
 	Voiced:
 		VoiceSet: MedicVoice
+	Selectable:
+		Priority: 5
 
 MECH:
 	Inherits: ^Soldier
@@ -357,6 +361,8 @@ MECH:
 		StandSequences: stand
 	Voiced:
 		VoiceSet: MechanicVoice
+	Selectable:
+		Priority: 5
 
 EINSTEIN:
 	Inherits: ^CivInfantry