Remove explicit default destructors (#157)

From CppCoreGuidelines:

Rule of Zero: C.20: If you can avoid defining default operations, do.
C.52: Use inheriting constructors to import constructors into a derived class that does not need further explicit initialization.
DRY forward and using declarations.
Miscellaneous:

Fix format.sh to output examples with normalised paths in sorted order.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

include/ftxui/screen/screen.hpp

@@ -10,9 +10,6 @@
 
 namespace ftxui {
 class Node;
-}
-
-namespace ftxui {
 using Element = std::shared_ptr<Node>;
 
 /// @brief A unicode character and its associated style.