Update visit_beamer_frame() frame title logic

[nolanking90]

Currently visit_beamer_frame(...) early returns None if no \frametitle command is found in the environment.

This PR updates visit_beamer_frame(...) to handle cases when there is an optional title argument e.g. \begin{frame}{title} or when no title is specified. A \frametitle command takes precedence here (as it does in latex). The optional environment argument is the fallback if no \frametitle is found, and if neither are present an untitled Frame symbol is still returned. The function has no path that can return None, but I have left the return type as an Option to match the pattern used at the call site. A helper function for extracting the title and a test to exercise the symbol name logic are provided.

resolves #1525

[pfoerster]

Thank you! 👍