Search box should use an alternate text for the submit button (so that it can be used effectively when images are disabled)

The reason is to conform to the guidelines:
"The alt (alternative) attribute should be used on every img (image) and area element and should be provided with an effective alternative text."

Right now we have in global.vm, line 31

<input class="button" value="$msg.get('panels.search.submit')" type="image" src="$xwiki.getSkinFile('icons/xwiki/search.png')"/>

We need to add an alt attribute. Also we should check whether the value attribute is necessary or not.