481: Changes to note input keyboard shortcuts introduced in Sibelius 4

Due to the addition of the new digital video and Playback window features in Sibelius 4, we made a number of improvements to the default keyboard shortcuts in the new version. We generally try and follow user interface conventions of both the operating systems we're running on and other comparable software; for years we had received requests from users that we should do what other music software does, and use the Space bar to start and stop playback. The way that inputting rests worked in previous versions was inconsistent and not particularly ergonomic for many users. For example, you could hit 0 on the keypad, then hit A on the main keyboard (or play a note on your MIDI keyboard!), and Sibelius would input a rest!