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/FilterTableHeader.cpp

@@ -58,6 +58,9 @@ FilterTableHeader::FilterTableHeader(QTableView* parent) :
     connect(this, SIGNAL(sectionResized(int,int,int)), this, SLOT(adjustPositions()));
     connect(parent->horizontalScrollBar(), SIGNAL(valueChanged(int)), this, SLOT(adjustPositions()));
     connect(parent->verticalScrollBar(), SIGNAL(valueChanged(int)), this, SLOT(adjustPositions()));
+
+    // Set custom context menu handling
+    setContextMenuPolicy(Qt::CustomContextMenu);
 }
 
 void FilterTableHeader::generateFilters(int number)