Common keyboard shortcuts

### Problem

There are many common shortcuts, like space to pause, left/right to skip ahead, M to mute and more that do not work, or do not work properly.

### Proposal

elementary/videos could support more of these commonly expected shortcuts, like:

- Space (or even K, like YouTube) to pause
- Left/Right to skip ahead
- M
- Maybe more?

Small sidenote:
The application *does* have a left and right shortcut, unfortunately they only work when the UI is visible by means of i.e moving the cursor. I will open a separate issue about that if needed.

### Prior Art (Optional)

Some other video players that have shortcuts like these:
- [VLC](https://www.videolan.org/vlc/)
- [MPV](https://mpv.io/)
- ['Video Player'/Showtime](https://apps.gnome.org/Showtime/) from the GNOME Project.