preserve filenames on sharing

[r10s]

this PR copies files to share, to give the original file name to other apps.

closes #2524

before

this is the baseline:

after

the filename is test.pdf - the extension is hidden by iOS, and shown as the type:

other tests

i tested also to set metadata instead of copying, this was fine in the "share preview", however, the final app again gets the wrong filename, see details below.

i tried that using all of activityViewControllerPlaceholderItem, LPLinkMetadata.originalURL, LPLinkMetadata.title, QLPreviewItem.title - none of them worked for me.

the gallery, however, still contains the scrambled names, copying them would require to copy all images on each swipe, that seems over the too. also, when sharing images, the name is barely anywhere displayed on the destination apps - and even the unscrambled name often looks scrambles.

still, user can share from delta chat and not from gallery to get the correct names. if ppl complain at scale, we can iterate :)