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

examples/component/CMakeLists.txt

@@ -10,6 +10,7 @@ example(gallery)
 example(homescreen)
 example(input)
 example(maybe)
+example(focus)
 example(menu)
 example(menu2)
 example(menu_entries)