Max extend: fix current extend after sheet size changed

[kirznernasta]

When SlidingSheet increases its height and currentExtent has value of maxExtent, after recalculation of all widget component's height the new value of maxExtent will be bigger than previous so the currentExtent won't be changed. So that's why SlidingSheet won't be with maximum extention as we expect. Because of than I've created a new varible previousMaxExtent in _measure() to check in _updateSnappingsAndExtent(), whether the previous currentExtent had the value of maxExtent, and if it's so I change its value to the new value of maxExtent.