ConfirmBox: avoid overflowing screen height when long text
ConfirmBox: avoid overflowing screen height when long text

Just like what's been done previously for InfoMessage.
Also tweak 'other_buttons' option (not yet used anywhere)
to allow multiple rows of such other buttons.

pazos/koreader,Frenzie/koreader,NiLuJe/koreader,mihailim/koreader,poire-z/koreader,poire-z/koreader,koreader/koreader,koreader/koreader,Hzj-jie/koreader,mwoz123/koreader,NiLuJe/koreader,Markismus/koreader,Frenzie/koreader