Alternate PR for Updates for buttons changes in core

Alternate PR to !104 (closed) for updates for input-to-button and styling changes in core in 5.31, including also that using button text to determine the clicked button doesn't work anymore.

Also sort of addresses #107 (closed) regarding the leaf disappearing which broke in an earlier version by just no longer using a leaf/custom icon.

Note there's changes both with and without popup forms enabled.

Note also the diff for the css file is maybe harder to read than just looking at the resulting file, which is now pretty simple.

It looks a little oversized in 5.30 on regular forms but it was a little messed up before anyway and also I think that was part of the problem the core button changes are trying to solve in the first place.

For shoreditch, it looks ok on popup forms but looks weird for regular forms, but that seems partly pre-existing, so would probably be ok when shoreditch does its updates. It looks a different kind of weird for regular forms in 5.30.

Shoreditch regular forms 5.30 (button is on next line/almost invisible):

Shoreditch regular forms 5.31: