Improved consistency of XMP handling

[radarhere]

#5076 (comment) is a user who would like to retrieve XMP strings from images. This can be done, but inconsistently between formats.

This PR updates JPEG, TIFF and PNG to create im.info["xmp"] like WEBP does.

Then, given that common location of XMP data, it makes sense to tidy up the code by adding getxmp() to the Image class, rather than duplicating it for each of supported formats. getxmp() will return an empty dictionary for other formats.

[bigcat88]

Since this change removed the _getxmp function - which I used in pillow_heif, could you at least add parsing for the NULL terminating byte in xmp? @radarhere

Info is here:

bigcat88/pillow_heif#69
AOMediaCodec/libavif#1333 - topic is big, but most important are last messages

Because now I need to either remove the leading byte, which will lead to the fact that if the image is opened and saved, the xmp will differ in a result by that one byte.

That will allows me to just remove getxmp from pillow_heif, otherwise I will need to copy old _getxmp code and use it..

[radarhere]

Ah, I had presumed I was just removing an unused internal method.

I've created #8171. Let us know if you have any feedback.