Unbold

1. unbold labels and folders. Was high time for that – otherwise folders look like they have »unread« items in them
2. explicitly unbold headers in settings, otherwise might be bold

Number 2 is actually one of the funniest issues I ever encountered. With current master, set the language to French. The headings will be bold …

Please review @owncloud/designers  (We also need to backport 48fb251 then)