Improve `gnus-signature' and `recover-this-file'

Also, use a yellow volatile highlight for `next-error'.
Change color of `diredp-ignored-file-name', `recover-this-file' and `w3m-bold'
Update highlight-line, region and show-paren-match
Nihil inherit property for `font-lock-regexp-grouping-backslash/construct' faces
Renew picture with latest changes in Leuven theme
Update region and add hl-tags-face
Improve highlight line and Org tags
Improve highlight and Org tags