Rework general usage of 'Italic' and 'Bold' and buttons

[ollimeier]

From Masaki:
about plus-button and minus-button in the text, + / - icon can be added for easy understanding?

From Mark:

the alt-key could use the ⌥ symbol too

also in some places the name of a button is italicized, some places not. And italic is also used for some others things in some places.
Maybe it’s good to have one specific kind of typographic treatment for buttons, so that it’s easily understandable.