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 <cyphar@cyphar.com>

frontend/ui/widget/inputtext.lua

@@ -707,6 +707,20 @@ function InputText:searchString(str, case_sensitive, start_pos)
     return char_pos
 end
 
+--- Return the character at the given offset. If is_absolute is truthy then the
+-- offset is the absolute position, otherwise the offset is added to the current
+-- cursor position (negative offsets are allowed).
+function InputText:getChar(offset, is_absolute)
+    local idx
+    if is_absolute then
+        idx = offset
+    else
+        idx = self.charpos + offset
+    end
+    if idx < 1 or idx > #self.charlist then return end
+    return self.charlist[idx]
+end
+
 function InputText:addChars(chars)
     if not chars then
         -- VirtualKeyboard:addChar(key) gave us 'nil' once (?!)