fix(xtext): move hexcolor_parse to its own file for the standalone xtext tests

master's new xtext_tests target compiles xtext.c on its own with only
GTK linked, so hexcolor_parse cannot live in util.c. Keep it in a
dependency-free hexcolor.c that both test targets compile, and register
it in the Visual Studio project for the Windows build.

Claude-Session: https://claude.ai/code/session_01Jxd4AknGzdAENADWAGHCzx

src/common/common.vcxproj.filters

@@ -98,6 +98,9 @@
     <ClInclude Include="userlist.h">
       <Filter>Header Files</Filter>
     </ClInclude>
+    <ClInclude Include="hexcolor.h">
+      <Filter>Header Files</Filter>
+    </ClInclude>
     <ClInclude Include="util.h">
       <Filter>Header Files</Filter>
     </ClInclude>
@@ -202,6 +205,9 @@
     <ClCompile Include="userlist.c">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="hexcolor.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
     <ClCompile Include="util.c">
       <Filter>Source Files</Filter>
     </ClCompile>