Default fallback image now modifiable via add_file_fallback_image fun…

[DBinaghi]

See #913

Default fallback image was not programmaticaly modifiable because of it using a constant; I've solved the problem by creating a new entry for the $_fallbackImages array.

[zerocrates]

This is a pretty reasonable solution to your issue, but I don't love the "generic" key that's being used here... maybe an asterisk * would be better?

I also think just for any possible other code that might have referenced it, it's best to leave the GENERIC_FALLBACK_IMAGE constant there.

[DBinaghi]

I see no problem in using "*" instead of "generic" (I'd kept "generic" as that was the key used before, I thought for some reasons somebody I had gone for that).
And also in keeping the constant GENERIC_FALLBACK_IMAGE, it'd hurt nobody. In fact, it can be used even here, just by moving its definition before the definition of the other fallback images.

[zerocrates]

To refer to a class constant within the same class, you need to use self::, so self::GENERIC_FALLBACK_IMAGE. The way you have it will cause a warning (and if you're not overriding that entry with a plugin, an error).

The line defining the constant is fine, but as a matter of style, could you move it to the top of the class, above all the static properties?