Remove os.system calls

[hejops]

Aside from file.convert and the following 2 commands,

• clear (tag.fix)
• mpv (play.pmp)

calls to os.system should be minimised:

• cat
• echo
• find
• ls / exa
• ncdu (file.mover)
• notify-send
• rm
• waitdie
• xdg-open
• xset (play.pmp)

Some of the above commands will be found in any typical Linux distribution, and
were used for QOL features (e.g. notifications, file cleanup) that I didn't
feel like implementing myself.

However, the following commands are unlikely to be universal, and should
be replaced:

• ncdu
• notify-send
• waitdie -- a personal script of mine that is a wrapper for tail -f
• xdg-open
• xset