HRangeScrollBar and VRangeScrollBar +/- handle backgrounds not shown upon initialization

[sjaehn]

The background of the two handles "+" and "-" are not shown in HRangeScrollBar and VRangeScrollBar when initialized and added to the main window. They only become visible (and stay visible) once the scrollbar is changed (e.g., dragged). Note, the labels "+" and "-" and all the rest is/are correctly shown all the time.

[sjaehn]

The problem is caused by HRangeScrollBar and VRangeScrollBar drawing to child widgets (Frames button1 and button2) surfaces upon call of update() (wich is called if the parent scrollbar is clicked, dragged, resized or changed in any way). However, the just drawn child widget surface is overwritten when the child widgets update() method is called. This happens upon composite widget (like HRangeScrollBar and VRangeScrollBar) initialization when the child widgets are added to the parent composite widget.

Solutions:

1. Always call update() to all composite widgets after the start (ugly solution).
2. Use a functional drawing solution (to be implemented, see roadmap).