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/sqlitetablemodel.h

@@ -30,7 +30,7 @@ public:
 
     void setQuery(const QString& sQuery, bool dontClearHeaders = false);
     QString query() const { return m_sQuery; }
-    void setTable(const QString& table);
+    void setTable(const QString& table, const QVector<QString> &display_format = QVector<QString>());
     void setChunkSize(size_t chunksize);
     void sort(int column, Qt::SortOrder order = Qt::AscendingOrder);
 
@@ -68,6 +68,7 @@ private:
     int m_iSortColumn;
     QString m_sSortOrder;
     QMap<int, QString> m_mWhere;
+    QVector<QString> m_vDisplayFormat;
     QVector<int> m_vDataTypes;
 
     /**