Jet\UI_button_save
Directly extends Jet\UI_button to represent a generic button for saving (anything).
The goal is a consistent UI. This means that all buttons of a certain function should have the same color and the same icon (and possibly the same text, but this is not a dogma). Therefore, for such buttons it is neither necessary nor appropriate to set the appearance class and icon. However, everything else can be set, of course.