Optimize inserts in vector and refactor const reference objects (#659)

Signed-off-by: German Semenov <GermanAizek@yandex.ru>
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/dom/gridbox.cpp

@@ -37,6 +37,8 @@ class GridBox : public Node {
     for (const auto& line : lines_) {
       x_size = std::max(x_size, int(line.size()));
     }
+
+    // Fill in empty cells, in case the user did not used the API correctly:
     for (auto& line : lines_) {
       while (line.size() < size_t(x_size)) {
         line.push_back(filler());