guiutil.cpp/.h: fix a -Wreorder compiler warning and make constructor for ToolTipToRichTextFilter explicit

src/qt/guiutil.cpp

@@ -244,7 +244,7 @@ void openDebugLogfile()
 }
 
 ToolTipToRichTextFilter::ToolTipToRichTextFilter(int size_threshold, QObject *parent) :
-    size_threshold(size_threshold), QObject(parent)
+    QObject(parent), size_threshold(size_threshold)
 {
 
 }

src/qt/guiutil.h

@@ -81,7 +81,7 @@ namespace GUIUtil
     {
         Q_OBJECT
     public:
-        ToolTipToRichTextFilter(int size_threshold, QObject *parent);
+        explicit ToolTipToRichTextFilter(int size_threshold, QObject *parent = 0);
 
     protected:
         bool eventFilter(QObject *obj, QEvent *evt);