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/bullet_physics_server.cpp

@@ -79,9 +79,7 @@ void BulletPhysicsServer3D::_bind_methods() {
 }
 
 BulletPhysicsServer3D::BulletPhysicsServer3D() :
-		PhysicsServer3D(),
-		active(true),
-		active_spaces_count(0) {}
+		PhysicsServer3D() {}
 
 BulletPhysicsServer3D::~BulletPhysicsServer3D() {}