add maxScroll method for xbmcgui.ControlTextBox

xbmcgui.ControlTextBox has method scroll(offset), but which does not supply method to figure out the max scroll-able offset, so codes using ControlTextBox won't do a good scroll work if it scroll too much it will looks the page can not be scroll down or up anymore, because it scroll too much, until user scroll up as many as which scroll over, it can get the scroll function back.

anyway, this commit try to add a method to get the max scroll-able offset of the textbox (with the current text content set), the result value can be used to prevent from scroll over the text end.

I can not compile xbmc, so please test it before merge to main branch.