Added pitch fly mode (#7817)

In pitch fly mode, you fly to the exact direction you are pointing at, using the forward key. Other move directions are also pitched accordingly.
It allows smoother and more complex movements.
Can be enabled/disabled by L key by default (set keymap_pitchfly in minetest.conf)

src/player.cpp

@@ -139,6 +139,7 @@ void Player::clearHud()
 void PlayerSettings::readGlobalSettings()
 {
 	free_move = g_settings->getBool("free_move");
+	pitch_fly = g_settings->getBool("pitch_fly");
 	fast_move = g_settings->getBool("fast_move");
 	continuous_forward = g_settings->getBool("continuous_forward");
 	always_fly_fast = g_settings->getBool("always_fly_fast");