Switch to rounded borders. (#175)

They are nicer.
It would be nice supporting a way for FTXUI to let the user chose what
they prefer instead.

src/ftxui/dom/border.cpp

@@ -12,7 +12,12 @@
 
 namespace ftxui {
 
-static wchar_t simple_border_charset[] = L"┌┐└┘─│┬┴┤├";
+static wchar_t simple_border_charset[] = L"╭╮╰╯─│┬┴┤├";
+
+// For reference, here is the charset for normal border:
+// L"┌┐└┘─│┬┴┤├";
+// TODO(arthursonzogni): Consider adding options to choose the kind of borders
+// to use.
 
 class Border : public Node {
  public:

src/ftxui/dom/text_test.cpp

@@ -57,9 +57,9 @@ TEST(TextTest, CJK) {
   Screen screen(6, 3);
   Render(screen, element);
   EXPECT_EQ(
-      "┌────┐\r\n"
+      "╭────╮\r\n"
       "│测试│\r\n"
-      "└────┘",
+      "╰────╯",
       screen.ToString());
 }
 
@@ -69,9 +69,9 @@ TEST(TextTest, CJK_2) {
   Screen screen(5, 3);
   Render(screen, element);
   EXPECT_EQ(
-      "┌───┐\r\n"
+      "╭───╮\r\n"
       "│测试\r\n"
-      "└───┘",
+      "╰───╯",
       screen.ToString());
 }
 
@@ -81,9 +81,9 @@ TEST(TextTest, CJK_3) {
   Screen screen(4, 3);
   Render(screen, element);
   EXPECT_EQ(
-      "┌──┐\r\n"
+      "╭──╮\r\n"
       "│测│\r\n"
-      "└──┘",
+      "╰──╯",
       screen.ToString());
 }