Add focusPosition[relative](x,y) (#280)

It allows when using inside a frame, to scroll the view toward a
particular position.

This resolves:
https://github.com/ArthurSonzogni/FTXUI/issues/125

include/ftxui/dom/elements.hpp

@@ -68,6 +68,8 @@ Decorator color(Color);
 Decorator bgcolor(Color);
 Element color(Color, Element);
 Element bgcolor(Color, Element);
+Decorator focusPosition(int x, int y);
+Decorator focusPositionRelative(float x, float y);
 
 // --- Layout is
 // Horizontal, Vertical or stacked set of elements.