Refactor background support
Refactor background support

Now, an existing file will *always* override any built-in images (which
should go mostly unnoticed by end users since input files should always
have an extension).

Also, the code now looks in the module's resources directory for any
image, not just builtin-arrow. This opens the possibility for dmgbuild
to provide multiple template images for use by end users. As a result,
the default is now no background.

This also deduplicates the code for image backgrounds a bit.

al45tair/dmgbuild