This commit borrows the "New" emblem from gnome-icon-theme to give us a
more modern looking emblem for overlaying on files and directories in
directory comparison.

data/icons/COPYING: Note the license terms for the new emblem
data/icons/COPYING_CCBYSA3: Add CC-BY-SA-3 license
data/icons/emblem-meld-newer-file.png: The 'new' emblem
data/ui/meldapp.ui: Add artwork attribution to About dialog