Support combining characters. (#121)

Modify the ftxui::Pixel. Instead of storing a wchar, store a
std::wstring. Now a single pixel can store multiple codepoints.
If a codepoint is of size <=0, it will be appended to the previous
pixel.

Only ftxui::text() is supported. ftxui::vtext support still needs to be
added.

This causes the following CPU and memory regression:
- Memory: Pixel size increases by 200% (16 byte => 48byte).
- CPU:    Draw/Second decrease by 62.5% (16k draw/s => 6k draw/s on 80x80)

Both regressions are acceptable. There are still two orders of magnitude
(100x) before the levels where performance/memory concerns begins.

This fixes: https://github.com/ArthurSonzogni/FTXUI/issues/109

include/ftxui/screen/screen.hpp

@@ -18,9 +18,16 @@ using Element = std::shared_ptr<Node>;
 /// @brief A unicode character and its associated style.
 /// @ingroup screen
 struct Pixel {
+  // The graphemes stored into the pixel. To support combining characters,
+  // like: a⃦, this can potentially contains multiple codepoitns.
+  // Required: character.size() >= 1;
+  std::wstring character = L" ";
+
+  // Colors:
   Color background_color = Color::Default;
   Color foreground_color = Color::Default;
-  wchar_t character = U' ';
+
+  // A bit field representing the style:
   bool blink : 1;
   bool bold : 1;
   bool dim : 1;