Port the plot text method to Qt, also test it by actually plotting something

The plot_text function from the cairo-methods are now ported
on the qt version. this patch moves around some code since
quite defines are already used and I didn't want to reinvent
the whell.

Original code used varargs, but I prefered to change it
, so now it receives just a reference to a QString object
and the string must be constructed before sending,
using the .arg methods.

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Make the text ignores transformations on the scene

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>