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.

scene/resources/physics_material.cpp

@@ -69,9 +69,3 @@ void PhysicsMaterial::set_absorbent(bool p_val) {
 	absorbent = p_val;
 	emit_changed();
 }
-
-PhysicsMaterial::PhysicsMaterial() :
-		friction(1),
-		rough(false),
-		bounce(0),
-		absorbent(false) {}