Update user guide icons

Update user guide icons

Some icons were converted to PNG but were named with a .gif extension.
On Windows these icons do not display correctly.
Replacing the icons to their original GIF format from the .ui plugin.

Change-Id: If8a9798c3470265609e4a1ebb1e1f57347af0d8a
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/12256
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>