Audio player remove improvements

[ScrubN]

Changes:

• Add X hotkey for removing the currently selected audio in the audio player.
  • I was a bit conflicted on this choice. I would have preferred del, but since it is used to clear hotkeys in the settings window, it was not an option. My next choice was D but I wanted to avoid conflicting hotkeys, even if they were from different windows. Eventually I settled on borrowing Blender's delete key, D.
• Stop playback if the removed file was the currently playing file
• Clear state related to the currently playing audio if the last audio was removed from the playlist.

If there are any changes you would like made to this PR, please let me know.