I've got it working. Right now, the beginning of the file looks like this
Qt Code: 
  1. <?xml version="1.0" encoding="UTF-8"?>
  2. <project>
  3.     <info project_name="mattMathilde" project_path="C:/Users/gldalar5/mattMathilde" pad_width="800" pad_height="600"/>
  4.     <storyboard padCount="2" folder="epi1">
  5.         <pad timing="50">
  6.             <line p1x="79" p1y="68" p2x="81" p2y="68" rgb="4293958640" width="11"/>
  7.             <line p1x="81" p1y="68" p2x="83" p2y="67" rgb="4293958640" width="11"/>
  8.             <line p1x="83" p1y="67" p2x="87" p2y="66" rgb="4293958640" width="11"/>
  9.             <line p1x="87" p1y="66" p2x="91" p2y="64" rgb="4293958640" width="11"/>
  10.             <line p1x="91" p1y="64" p2x="97" p2y="63" rgb="4293958640" width="11"/>
To copy to clipboard, switch view to plain text mode 
Thanks to your suggestions, the xml is much better and precise now.
The 'padCount' value was moved from the 'project' to the 'storyboard' tag, where it belongs. A project can have many storyboards, and each storyboard must know how many pads there are to the storyboard. My mistake.
The 'count' value in the 'pad' tag has been removed, since I don't use the count, and just iterate over lines in each pad.
Each line has a color AND a width, so I added a 'width' value. Another mistake.
And - finally - the idea of calling the pad-node 'pad' and the line-node 'line', is so obvious, that it almost hurts.
Thankyou for helping!