emacs: show: lazy part handling bugfix

emacs: show: lazy part handling bugfix

The lazy part handler had a bug that it allowed the button to be
toggled to be specified. During toggling it needs to save and restore
the text-properties for the button but it actually saved the text
properties at point rather than from the button.

In almost all cases this didn't matter as as point had the same text
properties as the button. However, it is a bug and did cause incorrect
behaviour in some cases: see id:87txhz14z6.fsf@qmul.ac.uk for details.