Qt4ProjectConfigWidget: Update build directory line edit on build

That line edit shows a red text if the directory does not exist,
building triggers a recheck and removes the red if the directory exists.

Change-Id: I50e87816a3e1b4ee9ef6ccb91e66eec8f7783242
Reviewed-by: Robert Loehning <robert.loehning@digia.com>

src/libs/utils/pathchooser.h

@@ -131,6 +131,7 @@ public:
     bool isReadOnly() const;
     void setReadOnly(bool b);
 
+    void triggerChanged();
 private:
     // Returns overridden title or the one from <title>
     QString makeDialogTitle(const QString &title);