Use the Noto fonts from the C++ build of the printer demo

This adds API to register a font by path from C++

api/sixtyfps-cpp/include/sixtyfps.h

@@ -19,6 +19,7 @@ LICENSE END */
 #include <algorithm>
 #include <iostream> // FIXME: remove: iostream always bring it lots of code so we should not have it in this header
 #include <chrono>
+#include <optional>
 
 namespace sixtyfps::cbindgen_private {
 // Workaround https://github.com/eqrion/cbindgen/issues/43
@@ -669,4 +670,18 @@ void run_event_loop()
     cbindgen_private::sixtyfps_run_event_loop();
 }
 
+/// Registers a font by the specified path. The path must refer to an existing
+/// TrueType font font.
+/// \returns an empty optional on success, otherwise an error string
+std::optional<SharedString> register_font_from_path(const SharedString &path)
+{
+    SharedString maybe_err;
+    cbindgen_private::sixtyfps_register_font_from_path(&path, &maybe_err);
+    if (!maybe_err.empty()) {
+        return maybe_err;
+    } else {
+        return {};
+    }
+}
+
 } // namespace sixtyfps

api/sixtyfps-cpp/include/sixtyfps_string.h

@@ -85,6 +85,9 @@ struct SharedString
     const char *begin() const { return data(); }
     const char *end() const { return &*std::string_view(*this).end(); }
 
+    /// \return true if the string contains no characters; false otherwise.
+    bool empty() const { return std::string_view(*this).empty(); }
+
     /// Creates a new SharedString from the given number \a n. The string representation of the
     /// number uses a minimal formatting scheme: If \a n has no fractional part, the number will be
     /// formatted as an integer.