extended controls

[idcrook]

For #17 adding UI elements (buttons and slider), handlers and settings options.

Have implemented things like volume UI and mute and a revisited stop remote command

volume UI does not yet do anything (does not track current volume setting nor send remote commands)

• shuffle remote control does not seem like it is working
• fast forward and rewind sort of work but do not really have a way to control them beyond starting them

[idcrook]

added settings options:

webui:
# ...
  show_player_extended: true  # volume+slider and mute
  show_player_shuffle: false
  show_player_seeking: false
  show_player_stop: true

show_player has to be set for any of these to have meaning

[idcrook]

this PR tweaks time format, so is related to #16

[idcrook]

Now has working volume controls, including the slider