Printing support #1525: print DB structure

New action and button for printing the database structure.

The data in the Database Structure tree widget is converted to HTML and set
in a document that can then be printed, opening the dialog as done for
other components.

src/MainWindow.h

@@ -300,6 +300,7 @@ private slots:
     void exportFilteredTable();
     void updateInsertDeleteRecordButton();
     void runSqlNewTab(const QString& query, const QString& title);
+    void printDbStructure();
 };
 
 #endif