[refactor] Split Database.cpp into db subdirectory

* Database.hpp is still the only external include file (moved to db/)
* The header is still uppercase to match its namespace
* db/internal.hpp is the shared header for the DB source files
* Added -Isrc/ compile flag for src-relative include paths
* Hoisted CHDR above CSRC in Makefile (it was bothering me)
* make clean now removes all objects in the subdirectories as well

src/BuddyManager.cpp

@@ -3,9 +3,9 @@
 #include "ChatManager.hpp"
 #include "PlayerManager.hpp"
 #include "BuddyManager.hpp"
-#include "Database.hpp"
+#include "db/Database.hpp"
 #include "ItemManager.hpp"
-#include "Database.hpp"
+#include "db/Database.hpp"
 
 #include <iostream>
 #include <chrono>