theming: Introduce image size fallback mode

This changes image size handling a little bit. An optional <fallback size="xxx" />
tag tells the generator to use different image size when an image for the current
image size has not been supplied from the album XML file.

This is effectively used for the legacy <nofullsize> tag fallback. More generally
this is useful for templates showing additional image sizes. Also saves space as
images not having the custom size supplied are not generated from the source image.

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