inputtext: add access methods for internal character list
inputtext: add access methods for internal character list

This allows for InputText wrappers (namely the Japanese keyboard which
needs to be able to apply modifiers to the character before the cursor)
to nicely access the character list.

Signed-off-by: Aleksa Sarai <b305c9b0212d6145eea67bbffd24e29cb79c759c@cyphar.com>

poire-z/koreader,koreader/koreader,koreader/koreader,NiLuJe/koreader,Frenzie/koreader,NiLuJe/koreader,Frenzie/koreader,poire-z/koreader