Recent Files menu

This adds a Recent Files menu, per popular request (e.g., #124 and #221 (closed)). Tested on Linux. Hopefully also works on Mac/Windows, but still needs to be tested there.

On Linux, the recent file list is stored in the recent_files file in the config dir, one file name per line, which is compatible with classic pd-l2ork. Persistent storage of the recent files list on Mac/Windows is in the defaults/registry (nrecent and recent1, recent2, ... keys, as suggested in the comments).