cmake-gui: Add "Building" category to freedesktop menu spec

Add "Building" to Categories parameters like suggested by the
specification [1] for better XFCE/GNome/Plasma menu integration.

[1] https://specifications.freedesktop.org/menu-spec/latest/apas02.html

Source/QtDialog/cmake-gui.desktop

@@ -7,6 +7,6 @@ Icon=CMakeSetup
 Terminal=false
 X-MultipleArgs=false
 Type=Application
-Categories=Development;
+Categories=Development;Building;
 StartupNotify=true
 MimeType=application/x-cmakecache;