Support alternative display formats in the Browse Data tab

This is a proof-of-concept or even a basic first implementation of a new
feature I'd like to have in DB4S which at the moment I call display
formats.

The idea here is to allow the user to change the data in the Browse Data
tab on a per column basis before displaying it. This means even though
the data is stored in format X in the database it can be shown in format
Y in the browser. This should be useful in cases where the original
format X is hard to read or just not useful in a particular case.

This first implementation allows the user to right click on the header
of a column and open a new dialog for setting the display format which
offers a (limited) list of pre-defined formats. The selected format is
then integrated into the SELECT statement which is sent to SQLite.

While it works, this draft implementation lacks a number of features.
Here are the most prominent ones I'm currently aware of:
* Data not editable (or only via the Edit Dialog) because it isn't
  transformed back yet.
* More display formats needed; maybe customizable ones, too.
* No indication in the UI for which columns a format has been set.
* Could _maybe_ be integrated into the import/export etc. for optional
  use.

src/MainWindow.h

@@ -38,6 +38,7 @@ public:
         Qt::SortOrder sortOrderMode;
         QMap<int, int> columnWidths;
         QMap<int, QString> filterValues;
+        QMap<int, QString> displayFormats;
 
         friend QDataStream& operator<<(QDataStream& stream, const MainWindow::BrowseDataTableSettings& object)
         {
@@ -45,6 +46,7 @@ public:
             stream << static_cast<int>(object.sortOrderMode);
             stream << object.columnWidths;
             stream << object.filterValues;
+            stream << object.displayFormats;
 
             return stream;
         }
@@ -56,6 +58,7 @@ public:
             object.sortOrderMode = static_cast<Qt::SortOrder>(sortordermode);
             stream >> object.columnWidths;
             stream >> object.filterValues;
+            stream >> object.displayFormats;
 
             return stream;
         }
@@ -98,6 +101,7 @@ private:
     QMenu *popupTableMenu;
     QMenu *recentFilesMenu;
     QMenu *popupSaveSqlFileMenu;
+    QMenu* popupBrowseDataHeaderMenu;
 
     QLabel* statusEncodingLabel;
     QLabel* statusEncryptionLabel;
@@ -207,6 +211,8 @@ private slots:
     void copyCurrentCreateStatement();
     void on_comboLineType_currentIndexChanged(int index);
     void on_comboPointShape_currentIndexChanged(int index);
+    void showDataColumnPopupMenu(const QPoint& pos);
+    void editDataColumnDisplayFormat();
 };
 
 #endif