Add descriptions for Cover Art
update grammar
Add cover art to 'what is new' list
Update picture of track dialog
add first description of Track Source feature
Fix spelling and minor typos
document supported cover image types
add human readable search feature
Merge branch 'cover-art' into manual

Conflicts:
	source/chapters/djing_with_mixxx.rst
Merge branch 'manual' of github.com:mixxxdj/manual into manual-updates
Small notes