playerctl: add settings to control which elements are displayed

[uninsane]

the defaults are to keep existing behavior, and display all fields.

new boolean options are added for:

• show-previous shows "<<" icon to rewind the player
• show-play-pause shows "||" or "|>" icon to play/pause the player
• show-next shows ">>" icon to advance the player
• show-name shows the media name (e.g. the artist + title) and the number of players

these are especially useful for narrow screens, like mobile phones, where a user may wish to disable all but "show-play-pause" in order to fit size limitations of the panel. see below for an example (the chevron in the control-settings menu was also removed to make this fit: i can open a PR to make that an option too if this type of change is appreciated).