Debugger: Fix using the same watcher number twice.

Counting the watcher number in a global scope, so they don't get
reinitialized incorrectly with a new engine.

Change-Id: Ifd9efb320e2c0f20b2e2845348a98961f72bf28c
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/debugger/watchhandler.h

@@ -257,8 +257,6 @@ private:
     DebuggerEngine *m_engine; // Not owned.
     SeparatedView *m_separatedView; // Owned.
 
-    int m_watcherCounter;
-
     bool m_contentsValid;
     bool m_resetLocationScheduled;
 };