Changes :yank's flag arguments to a positional "what" argument specifying the object to be yanked. Including "selection" as a possibility allows for the replacement of :yank-selected with :yank selection.
This also makes it work for QtWebEngine. It also seems to fix #1050 though I'm not 100% sure why.
This renames tests/integration to tests/end2end and moves some files to tests/end2end/fixtures.