Change X11 Window WM_CLASS to 'dosbox-staging' to avoid conflicts with
upstream DOSBox, which might be provided by package manager.
Update .desktop file to specify WMClass to be correlated with
dosbox-staging process (even if executable file conflicts with dosbox
upstream).
Update contrib/icons/Makefile to generate 'hicolor' directory structure.
Remove pre-rendered icons, we don't need them any more. Vector designs
for small icons are grouped in small-svg directory - these are slightly
less detailed and adjusted to render more clearly in specific sizes.
