[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/db/internal.hpp

@@ -0,0 +1,15 @@
+#pragma once
+
+#include "db/Database.hpp"
+#include <sqlite3.h>
+
+#if defined(__MINGW32__) && !defined(_GLIBCXX_HAS_GTHREADS)
+    #include "mingw/mingw.mutex.h"
+#else
+    #include <mutex>
+#endif
+
+extern std::mutex dbCrit;
+extern sqlite3 *db;
+
+using namespace Database;