[1.0] Spacebar for play/pause

The current stable version has no default shortcut for playing the score. The missing shortcut is significant because playback is a frequent action in notation software. For a detailed discussion see: #7425: Space bar for play/pause

Spacebar for play/pause is already available in the trunk, but I initially left it out of the stable branch because I didn't want to add a behavioral change to a bug-fix release. (Most users of MuseScore are used to pressing spacebar to add rests). Since the next release of the stable branch is more than a bug-fix release and we are bumping the version number to 1.0 I think it is time to add this to the stable branch.

The attached patch changes two lines of code in actions.cpp