It' is not major request but could be very usefull when there is a lot tracks and someone would like to push them all to just one playlist.

So or "Select All" button or 'CTRL+A' solution can be very handy

Keyboard navigation in the media library and file manager are one of the things I hope to improve on in coming versions, mostly for accessibility reasons but also for convenience.  A select-all hotkey is a great idea.

Meantime, while it's admittedly not nearly as convenient, remember that you can click the first track, scroll to the bottom, then shift+click the last track to select all.