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.

drivers/coremidi/midi_driver_coremidi.cpp

@@ -112,9 +112,7 @@ PackedStringArray MIDIDriverCoreMidi::get_connected_inputs() {
 	return list;
 }
 
-MIDIDriverCoreMidi::MIDIDriverCoreMidi() :
-		client(0) {
-}
+MIDIDriverCoreMidi::MIDIDriverCoreMidi() {}
 
 MIDIDriverCoreMidi::~MIDIDriverCoreMidi() {
 	close();