stbt.wait_for_match per-frame debug output includes template filename

For long-running `wait_for_match` calls (for example with
`timeout_secs=120`) I find it really annoying to have to scroll up in
the log to find what image it's searching for. This will make the logs
much more verbose (particularly because the image path is an absolute
path†) but that's a problem that can be solved in the log-viewing tools
(turn off line-wrapping in `less`; we'll have to come up with a solution
when viewing the logs in the browser.)

† Perhaps we can truncate the path at the boundary of the git
repository, like `stbt batch run` does when reporting the test script's
filename.
fixup! stbt.wait_for_match per-frame debug output includes template filename

(Rename MatchResult.template to MatchResult.image_name:

 * So far we use the word "template" in documentation and private
   function parameters, but always "image" in public functions'
   parameters.
 * We have to store the filename; we can't store the image itself
   because we can't get the filename from the OpenCV image (which is
   merely a numpy array).
)