Use shared_ptr instead of unique_ptr for elements.

This allow users to pass it into initializer list. Then clang-format
will produce 'acceptable' indentations.

This fixes:
https://github.com/ArthurSonzogni/FTXUI/issues/18

tutorial/build-with-cmake.md

@@ -18,13 +18,13 @@ main.cpp
 int main(int argc, const char *argv[]) {
   using namespace ftxui;
   auto document =
-    hbox(
+    hbox({
       text(L"left") | bold | border,
       text(L"middle") | flex | border,
-      text(L"right") | border
-    );
+      text(L"right") | border,
+      });
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
-  Render(screen, document.get());
+  Render(screen, document);
 
   std::cout << screen.ToString();
 

tutorial/build-with-nxxm.md

@@ -18,13 +18,13 @@ main.cpp
 int main(int argc, const char *argv[]) {
   using namespace ftxui;
   auto document =
-    hbox(
+    hbox({
       text(L"left") | bold | border,
       text(L"middle") | flex | border,
-      text(L"right") | border
-    );
+      text(L"right") | border,
+      });
   auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
-  Render(screen, document.get());
+  Render(screen, document);
 
   std::cout << screen.ToString();
 

tutorial/tutorial.md

@@ -132,17 +132,19 @@ Display a vertical or horizontal line to visually split the content of a
 container in two.
 
 ~~~cpp
-border(hbox(
-  vbox(
-    text(L"left top"),
-    text(L"left bottom")
-  ),
-  separator(),
-  vbox(
-    text(L"right top"),
-    text(L"right bottom")
-  )
-));
+border(
+  hbox({
+    vbox({
+      text(L"left top"),
+      text(L"left bottom"),
+    }),
+    separator(),
+    vbox({
+      text(L"right top"),
+      text(L"right bottom"),
+    }),
+  })
+);
 ~~~
 
 ~~~bash
@@ -213,11 +215,11 @@ An horizontal flow layout is implemented by:
 
 #### Examples
 ~~~cpp
-  hbox(
+  hbox({
     text(L"left") | border ,
     text(L"middle") | border | flex,
-    text(L"right") | border
-  );
+    text(L"right") | border,
+    });
 ~~~
 ~~~bash
 ┌────┐┌─────────────────────────────────────────────────────────────────┐┌─────┐
@@ -226,11 +228,11 @@ An horizontal flow layout is implemented by:
 ~~~
 
 ~~~cpp
-  hbox(
+  hbox({
     text(L"left") | border ,
     text(L"middle") | border | flex,
-    text(L"right") | border | flex
-  );
+    text(L"right") | border | flex,
+    });
 ~~~
 ~~~bash
 ┌────┐┌───────────────────────────────────┐┌───────────────────────────────────┐