Fix up include headers, try to reduce dependencies

Ensure that all headers are self-sufficient instead of relying on included
headers to include headers for execution.

Dengine.vcxproj.filters

@@ -89,5 +89,8 @@
     <ClInclude Include="src\include\Dengine\Debug.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="src\include\Dengine\Assets.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
   </ItemGroup>
 </Project>