Move filter header from window to table widget and show sort indicators

Move the FilterTableHeader object from the MainWindow to the
ExtendedTableWidget class because it actually is a part of the latter,
not the former.

Show sort order arrows in the table header of the Browse Data tab after
clicking it.

src/MainWindow.h

@@ -19,7 +19,6 @@ class DbStructureModel;
 class QNetworkReply;
 class QNetworkAccessManager;
 class QTreeWidgetItem;
-class FilterTableHeader;
 
 namespace Ui {
 class MainWindow;
@@ -69,7 +68,6 @@ private:
 
     SqliteTableModel* m_browseTableModel;
     SqliteTableModel* m_currentPlotModel;
-    FilterTableHeader* m_tableHeader;
     QMenu *popupTableMenu;
     QMenu *recentFilesMenu;