Now labels and HTML boxes can contain relative URLs. If we don't have
a base URL, the project file will be used as a base URL (closes#7236).
Remove the exception for the labels where the images where not
loaded (unless in in PDF or image mode). It was because of a crash. Qt didn't
like having the HTML loading to be done synchronously during painting.
Fix a leak when rendering labels.
label font and margins when rendering as HTML.
This allows interactive choice of font, margins and colors and
avoids the need to manually set these with CSS within the
label HTML code.
