Only enable filter actions when Extended Table has filter lines

When the filter lines have not been generated for a Extended Table
(currently the case for Execute SQL tables), the filter actions make no
sense, so they are disabled.

See issue #1469

src/ExtendedTableWidget.cpp

@@ -213,6 +213,11 @@ ExtendedTableWidget::ExtendedTableWidget(QWidget* parent) :
         copyWithHeadersAction->setEnabled(enabled);
         copyAsSQLAction->setEnabled(enabled);
 
+        // Hide filter actions when there isn't any filters
+        bool hasFilters = m_tableHeader->hasFilters();
+        filterAction->setVisible(hasFilters);
+        filterMenu->menuAction()->setVisible(hasFilters);
+
         // Try to find out whether the current view is editable and (de)activate menu options according to that
         bool editable = editTriggers() != QAbstractItemView::NoEditTriggers;
         nullAction->setEnabled(enabled && editable);

src/FilterTableHeader.h

@@ -15,6 +15,7 @@ class FilterTableHeader : public QHeaderView
 public:
     explicit FilterTableHeader(QTableView* parent = nullptr);
     virtual QSize sizeHint() const;
+    bool hasFilters() const {return (filterWidgets.count() > 0);};
     
 public slots:
     void generateFilters(int number, bool showFirst = false);