adds alt,href and title to image insert dialog

this adds "alt" "href" and "title" field to insert image dialog.
"alt" is obviously added to img tag and if "href" is not empty then the image is wrapped with an anchor with "href" and "title" set.

fixes #648 #472 #353 with alt tag
and #226 #212 with an anchor.

I would also like to add an "edit" button to the image popover witch will show this dialog with "url" and "file" inputs hidden so this parameters can be edited without the need to remove and re-insert image but this would conflict with #690 because of the popover changes so I will hold on with this.

cheers.