Fixed memory leak and added "append" keyword for margins in SCORINFO

After glancing through the source code, I noticed that when ScoreMargin::CommandBlock::ParseBlock is called (i.e. ScoreMargin::Parse), it clears the array of commands without deleting the command objects themselves like it does in ScoreMargin::CommandBlock::Clear. Without deleting these commands, and no way of referencing them again, it can cause a memory leak. This MR fixes the memory leak by calling the aforementioned member function to clear everything.

I also added an optional append keyword that comes after the name of the margin being edited (e.g. MainHeader append { }) so that any commands that were in the margin before aren't deleted, allowing users to make edits to the margins without clearing everything.