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/sessionengine.cpp

@@ -30,6 +30,7 @@
 #include "sessionengine.h"
 #include "breakhandler.h"
 #include "watchhandler.h"
+#include "debuggerconstants.h"
 
 #include <utils/qtcassert.h>
 
@@ -64,6 +65,12 @@ void SessionEngine::saveSessionData()
     breakHandler()->saveSessionData();
 }
 
+unsigned SessionEngine::debuggerCapabilities() const
+{
+    return DebuggerEngine::debuggerCapabilities()
+            | AddWatcherCapability | WatchpointCapability;
+}
+
 } // namespace Internal
 } // namespace Debugger