Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

modules/bullet/area_bullet.cpp

@@ -44,18 +44,7 @@
 */
 
 AreaBullet::AreaBullet() :
-		RigidCollisionObjectBullet(CollisionObjectBullet::TYPE_AREA),
-		monitorable(true),
-		spOv_mode(PhysicsServer3D::AREA_SPACE_OVERRIDE_DISABLED),
-		spOv_gravityPoint(false),
-		spOv_gravityPointDistanceScale(0),
-		spOv_gravityPointAttenuation(1),
-		spOv_gravityVec(0, -1, 0),
-		spOv_gravityMag(10),
-		spOv_linearDump(0.1),
-		spOv_angularDump(1),
-		spOv_priority(0),
-		isScratched(false) {
+		RigidCollisionObjectBullet(CollisionObjectBullet::TYPE_AREA) {
 
 	btGhost = bulletnew(btGhostObject);
 	reload_shapes();