Core/Utils: Migrate further to Utils::Id

The coreplugin/id.h header is kept for downstream for now.

Change-Id: I8c44590f7b988b3770ecdc177c40783e12353e66
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/dialogs/settingsdialog.cpp

@@ -64,6 +64,8 @@ const int kMaxMinimumHeight = 250;
 static const char pageKeyC[] = "General/LastPreferencePage";
 const int categoryIconSize = 24;
 
+using namespace Utils;
+
 namespace Core {
 namespace Internal {