Switching slider placeholder to inline

When a <gallery type="slider"> is the first element in RTE, IE is unable to place the cursor before. This is because in contenteditable, we have a <p> with a placeholder <img> to represent the slider and the <img> is set to display:block. By setting the image to display:inline, IE will naturally allow placement of the cursor before and after the image, within the paragraph. Unfortunately, this is not true WYSIWYG because the slider actually renders as display:block. I ran this option by Trevor Bolliger and compared it to the current handling of gallery (which also deviates from WYSIWYG) and he approved this fix. I can't wait for VisualEditor which will solve these types of problems with block slugs. @kflorence or @inez, please review.