It appears that GTK+/GNOME don't have an icon with
"help-browser" ID anymore, but we have exactly the icon
that is needed for two menu entries.

Also, the menu entry for "Search and Run..." was using
a system icon for finding stuff, which looked wrong when
used with a symbolic theme.

We could come up with something overly clever like
a dedicated CLI/terminal icon, but people would expect
a magnifier instead (judging by what Blender does)
and that's what we already use for the Zoom tool.
Hence the lazy fix.