debugger: make watchers part of the Locals and Watchers view resizable

Still too hidden. Probably the 'Always adjust' settings should be persistent.

src/plugins/debugger/debuggermanager.h

@@ -259,6 +259,7 @@ public slots: // FIXME
         { showDebuggerOutput(LogDebug, msg); }
     void ensureLogVisible();
     void updateWatchersWindow();
+    void updateWatchersHeader(int section, int oldSize, int newSize);
     void activateBreakpoint(int index);
 
 //private slots:  // FIXME