Debugger[CDB]: Do not evaluate watch expressions at all.

setError() on them instead (should there be any from
a session mixup). Give the session engine watcher
capabilities and ensure 'Remove watch item' is enabled
for the session engine by checking the debugger state.

Reviewed-by: hjk

src/plugins/debugger/cdb/cdbengine.cpp

@@ -705,7 +705,7 @@ void CdbEngine::updateWatchData(const WatchData &incomplete, const WatchUpdateFl
 
     if (incomplete.iname.startsWith("watch.")) {
         WatchData watchData = incomplete;
-        evaluateWatcher(&watchData);
+        watchData.setError(tr("<not supported>"));
         watchHandler()->insertData(watchData);
         return;
     }