call update() to trigger a repaint on changes
add IconComboBox

This QComboBox derived class overrides sizeHint() and
minimumSizeHint() as well as the paintEvent() function
to draw only the icon without text. If shown, the width
of the popup menu needs to be increased in order to show
all icons with text. Code could be nicer and more documented.
add prelimiary Label and Path property widet
fix popup size of IconComboBox
create undo-actions from QUndoStack
IconComboBox: add api documentation
first version of line property widget
add PaintHelper::drawPath()
use PaintHelper to draw the path outline
Node: trigger repaint if the style changed
testing: change the Document style
attempt to fix Style::innerLineWidth, maybe still wrong