Allow to hide menu icons

I tested this for three weeks, and when I wanted to revert to the
old behavior, I was really shocked how cluttered the menus look
with icons :)

There are only two places, where I initally missed the icons:
- "Classic" KDE menu
- Konqueror Bookmarks menu

For those places, we could request icons by using the QAction
property, but this probably has to discussed with users.

Actually, I now like the classic menu not showing icons,
because it opens much faster without them. The bookmark
menu never was a problem speedwise.

See http://svn.reviewboard.kde.org/r/5609/