dlgtrackinfo - popup menu of cover: rename "reload from metadata" to "reload"
As it is using the search algorithm - it will try to reload the cover as normally we do (firstly looking in the id3tag and then in the trackdir)
it dissociates the coverArtReloading process to the button "reload track metadata from file"

When an user clicks on "reload track metadata from file", Mixxx must not touch the cover art
Remove warning boxes
- as all fields are always overwritten when the user choose "reload" them,
it did not make sense having warning boxes (to warn about cases of overwriting) just for covers...
Rename method - from reloadEmbeddedCover() to slotReloadCover()
Remove unused member: m_sLoadedMd5Hash
Removing unused method CoverArtCache::getHashOfEmbeddedCover()