Allow opening files made in newer versions

[fernandomartin777]

Currently when we try to open a file saved in a newer version of Musescore saying that it cannot read the file and thta it was saved using a newer version. True, it's more secure this way because an older version will not corrupt newer features embedded in a file saved in a newer version. However, this feature has a very serious caveat: It prevents us from using beat and testing versions regularly. It often happens to me that I want to try a version still in development and give feedback to the devs if necessary, but I won't spend much of my time only with "testing" files. If I'm going to spend a significant time testing a new version I'm going to use it in a real piece I'm working on. But then if I do so I can't open that piece in the current stable version anymore. If you remove that warning message or put an option on it allowing us to proceed anyway, you can have more people testing because we'll be able to test beta and nightly versions (some of them already in a stable way) using real works that can later be opened in the current installed version.