Use an icon to represent expanded/collapsed state

Use an icon to represent expanded/collapsed state

Use a listview icon to represent expanded/collapsed state, rather than
puttting '[+]'/'[-]' in front of the category name

Just use the item icon directly to represent expanded/collapsed state,
rather than using a state icon as otherwise we have empty space where the
item icon would be, when it's size is being used for indenting.

The icons were made by tracing the previously used .bmp in Inkscape to
produce a .svg, then converting that to an icon with ImageMagick using
'convert -filter point -background transparent -define icon:auto-resize'

v2:
Drop execute mode (Achim Gratz)