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).

[nwg-piotr]

Your PR misses necessary additions to config.py and language files. We don't expect users to edit configs manually.

[uninsane]

i can add the new keys to en_US.json and dig into config.py. i expect i would be touching the glade files (nwg_panel/glade/config_playerctl.glade), which will be new for me. i'm guessing you don't edit those by hand: is the workflow something like me installing glade and then running glade config_playerctl.glade to edit that graphically?

before i do those things, do you agree with the direction of this PR? i was split between separate config options for show-previous and show-next v.s. one unified show-navigation option, for example. or taking all the elements here (previous/play-pause/next/cover-image/media-name) and exposing just a single list-type config option like you do with the "components" in the controls module.

[nwg-piotr]

Yes, you open and edit glade files with Glade. :)

"Show navigation" may be ambiguous, but yes, leaving one of the two buttons while the second is turned off makes little sense. We just need a better name, maybe "Show back/forward" or something? Use Gtk.CheckButton for each option.

do you agree with the direction of this PR?

I'll probably always use all buttons, but who knows, maybe someone else is going to like it?