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/constraint_bullet.cpp

@@ -37,10 +37,7 @@
 	@author AndreaCatania
 */
 
-ConstraintBullet::ConstraintBullet() :
-		space(nullptr),
-		constraint(nullptr),
-		disabled_collisions_between_bodies(true) {}
+ConstraintBullet::ConstraintBullet() {}
 
 void ConstraintBullet::setup(btTypedConstraint *p_constraint) {
 	constraint = p_constraint;