Added getxmp() method

[UrielMaD]

Fixes #5076

I propose to return an object with tags names and its values because it would get easier for the user to find what he's looking for, instead of giving the tag number as the getexif() function.

The tags returned are the ones that came in the XML tree contained in the image, so there's no need for having a tags table.

Changes proposed in this pull request:

Feature for retrieving XMP tags from image contained in an object.
This is how it the feature looks

from PIL import Image
im = Image.open("url.jpg")
print(im.getxmp())

and the resultant object:

[UrielMaD]

@radarhere changes accepted!, you're right, it sounds clearer. Thank you.

[hugovk]

Thanks! I'll add this to the release notes.

[radarhere]

#5376 moved this method into JpegImageFile, since the method fetches data from applist, which is a JpegImageFile property.

[radarhere]

Note that in Pillow 8.3.0, the output of getxmp() has changed, due to #5455