#9745: Icon button new
a step towards fixing #9745
Adds an extra parameter to specify the size of the icon to be created by (gimp_icon_button_new) and (prefs_button_add) Some of which I changed to a bigger size, see vid. Has the potential to detect the custom icon size set by the user. However, that depends on this MR being accepted and others. And I suspect a parameter option is probably good enough for these.
