Add support for beatmap link parsing

[felipemarins]

Currently, if you paste a beatmap link into the beatmap ID text box, it just deletes all non-number characters and ends up leaving the beatmapset ID mixed up with the beatmap ID. In this pr, I turned it into an ExtendedLabelledTextBox with a regex filter on the SimulateScreen and BeatmapLeaderboardScreen classes to check for beatmap links and extracting the correct ID from them.

[tsunyoku]

The Path.Exists logic also needs revisiting, I'm not really sure what it's trying to do. Please ensure that the existing support for downloading a .osu file we don't have still works.