lua : add a function to remove an image from the database

dt.database.delete(image)

or

image:delete()
lua : add a function to move an image

dt.database.move_image(film, image)
film:move_image(image)
image:move(film)

this moves the image file, the associated XMP and the XMP for
the duplicates to the directory pointed by the film
lua : add a function to copy an image

dt.database.copy_image(image,film):image
image:copy(film):image
film:copy(image):image
[l10n] update Greek translation
Updated Dutch translation
Merge pull request #441 from MRIG/master

Updated Dutch translation
Merge pull request #439 from jinxos/gr_translation

[l10n] update Greek translation
Add support for audio notes playback ...

... by using an external application like 'aplay'.
This commit also stores the presence of .txt and .wav files in image
flags and updates these in the startup crawler. So if you no longer see
your text overlay: run the crawler (at least) once.