stbt-templatematch: use sensible, canonical variable names

This commit attempts to make the variable names used for the two image
file inputs and their corresponding numpy arrays, which previously were
mixed up and confusing.

For example, the file name of the template was stored in the variable
`template_image`, and then the image itself stored in `template`. Yet
`image` is the variable containing the `full_frame` numpy array, and was
given as the value to MatchResult.frame, and the not-yet used but optional
parameter MatchResult.image would (theoretically) be `template_image`.
Most confusing is the call to `stbt._match`, which had `image`, `template`
and `template_image` as arguments!

To make this much less confusing, I've adopted the naming convention used
by stbt templatematch's verbose debug output, where the input frame from
the video source is called `source` and the `template` is called `template`
and further clarified them with `_image` and `_file` suffixes.
stbt-templatematch: create MatchResult with `image` to avoid warning

Commit f51f6487 introduced the `image` parameter to the MatchResult class,
and added a DeprecationWarning which was invoked whenever a MatchResult
was created without `image` being set. However, the `image` parameter
wasn't added to stbt-templatematch, and so the verbose DeprecationWarning
came up every time you ran it.

Note that I could have added `image=template_image` and it would have
been valid, however this would print out "image=<Custom Image>", which
is pretty useless.