Changed history API in model, no longer does it set/return entire
std::vector of std::string at once.
Instead, now there is way to get/set any item or get/set the size of
history itself.
Complexity/speed is same (+super small function call overhead) but it's
clearer and doesn't force user to pack their strings into a vector
before giving them to us.
It also doesn't modify the history index so call to readHistory() is needed
to make newly added items be last ones.
This all is of course just in case someone wants own history instead of
ours via ECO_HISTORY option.