Refactoring: move GUID operations out of Joystick

- SDL_JoystickGUID -> SDL_GUID (though we retain a type alias)
- Operations for GUID <-> String ops are now in
  src/SDL_guid.c and include/SDL_guid.h
- The corresponding Joystick operations delegate to SDL_guid.c
- Added test/testguid.c

VisualC/SDL/SDL.vcxproj

@@ -234,6 +234,7 @@
     <ClInclude Include="..\..\include\SDL_filesystem.h" />
     <ClInclude Include="..\..\include\SDL_gamecontroller.h" />
     <ClInclude Include="..\..\include\SDL_gesture.h" />
+    <ClInclude Include="..\..\include\SDL_guid.h" />
     <ClInclude Include="..\..\include\SDL_haptic.h" />
     <ClInclude Include="..\..\include\SDL_hints.h" />
     <ClInclude Include="..\..\include\SDL_hidapi.h" />
@@ -367,6 +368,7 @@
     <ClInclude Include="..\..\src\SDL_assert_c.h" />
     <ClInclude Include="..\..\src\SDL_dataqueue.h" />
     <ClInclude Include="..\..\src\SDL_error_c.h" />
+    <ClCompile Include="..\..\src\SDL_guid.c" />
     <ClInclude Include="..\..\src\SDL_hints_c.h" />
     <ClInclude Include="..\..\src\SDL_internal.h" />
     <ClInclude Include="..\..\src\SDL_list.h" />

VisualC/SDL/SDL.vcxproj.filters

@@ -231,6 +231,9 @@
     <ClInclude Include="..\..\include\SDL_gesture.h">
       <Filter>API Headers</Filter>
     </ClInclude>
+    <ClInclude Include="..\..\include\SDL_guid.h">
+      <Filter>API Headers</Filter>
+    </ClInclude>
     <ClInclude Include="..\..\include\SDL_haptic.h">
       <Filter>API Headers</Filter>
     </ClInclude>
@@ -833,6 +836,7 @@
     <ClCompile Include="..\..\src\SDL_assert.c" />
     <ClCompile Include="..\..\src\SDL_dataqueue.c" />
     <ClCompile Include="..\..\src\SDL_error.c" />
+    <ClCompile Include="..\..\src\SDL_guid.c" />
     <ClCompile Include="..\..\src\SDL_hints.c" />
     <ClCompile Include="..\..\src\SDL_list.c" />
     <ClCompile Include="..\..\src\audio\SDL_audio.c">