[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/CustomCommands.cpp

@@ -5,7 +5,7 @@
 #include "NPCManager.hpp"
 #include "MobAI.hpp"
 #include "ItemManager.hpp"
-#include "Database.hpp"
+#include "db/Database.hpp"
 #include "TransportManager.hpp"
 #include "MissionManager.hpp"