Button AHAH needs to work for button too

[7:45pm] goba: eaton: I looked at it and it looks reasonable, although #executes_submit_callback is not exactly a well known key, it helps us avoid repeating submit, button and image_button here... your opinion?
[7:45pm] • eaton accidentally clicks that and joins the 'executes_submit_callback' channel on freenode. heh.
[7:46pm] eaton: goba: Yeah, it's basically just our internal identifier for elements that submit forms, regardless of their HTML element type. Hmmm.
[7:47pm] eaton: goba: it's the right thing to check for, and if there's a problem, it's the fact that that #flag should probably have a shorter and more meaningful name.
[7:47pm] goba: hehe
[7:47pm] goba: I wonder whether all will work as done in the patch with form_expand_ahah()
[7:48pm] goba: everything is ready to handle buttons and image buttons when used this way?
[7:48pm] goba: this is the only missing part?
[7:48pm] eaton: It should be OK. They submit their data the same way...
[7:48pm] • eaton will take a closer look to double-check, bit it looks right.
[7:48pm] eaton: "It looks right"... what a dangerous phrase, heh

Took a closer look and tested it with other AHAH callback buttons with no breakage. In the future, we should probably replace #executes_submit_callback with #is_a_button or something like that; that's really what it means, and submit callbacks have nothing to to with it. That's a to-do for Drupal 7, though. This is a good patch that fixes an oversight in the image button handling.

Thanks, committed.