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/gdnavigation/rvo_agent.cpp

@@ -36,8 +36,7 @@
 	@author AndreaCatania
 */
 
-RvoAgent::RvoAgent() :
-		map(nullptr) {
+RvoAgent::RvoAgent() {
 	callback.id = ObjectID();
 }