Fix image widgets display under IE

There are several issues with images management in nuxeo:

• when displaying an image without value, the context may be reset and produce strange bugs (i'll find the jira issue) so no image should be displayed in that case. this is currently handled by adding cumbersome "rendered" clauses on nxu:grapghicImage or nxh:graphicImage tags.
• when setting a width and height (for instance when referencing potential widget properties), empty tags are generated => IE does not display these images well.

As components nxu:graphicImage and nxh:graphicImage were added some time ago as wrapper tags, they could be customized to handle these needs (currently still using the default JSF implementation).