Bitcoin-Qt: massive header and cpp cleanup

- try to enforce the same style to all Qt related files
- remove unneeded includes from the files
- add missing Q_OBJECT, QT_BEGIN_NAMESPACE / QT_END_NAMESPACE
- prepares for a pull-req to include Qt5 compatibility

src/qt/overviewpage.h

@@ -3,10 +3,6 @@
 
 #include <QWidget>
 
-QT_BEGIN_NAMESPACE
-class QModelIndex;
-QT_END_NAMESPACE
-
 namespace Ui {
     class OverviewPage;
 }
@@ -15,6 +11,10 @@ class WalletModel;
 class TxViewDelegate;
 class TransactionFilterProxy;
 
+QT_BEGIN_NAMESPACE
+class QModelIndex;
+QT_END_NAMESPACE
+
 /** Overview ("home") page widget */
 class OverviewPage : public QWidget
 {