add images to quotes

[r10s]

quotes recently became much more useful by the "jump to quote" feature.

what is still missing, is enriching the quote eg. by the image if an image was quoted.

to get rich content, one should check dc_msg_get_quoted_text() to check if there is a quote at all. if so, use dc_msg_get_quoted_msg() to get the full, quoted message object - that may be null if the message is not availalbe. however, if that is non-null, you can check viewtype and file to get the image as usual.

this is how that looks like on ios - android is similar:

it may also make sense to display other rich content, as documents, however, displaying images have by far the largets impact - so maybe just do that for now and move on to other things with larger impact :)