Create high-DPI variants for more of the GEF icons

This converts the following icons to PNG and adds a 192 DPI version for:
- folder_closed
- move_down_disabled
- move_up_disabled
- next_nav
- palette_layout
- palette_view
- prev_nam
- zoom_in
- zoom_out

The following icons are not used and have been removed:
- palette_left
- palette_right

Most of the SVG files have been taken from the Eclipse platform project,
in order to remain consistent with the rest of the IDE.

See #465