* add JavaScript to submit the changes before storing them
* add input validation
* fix settings page loading in case of networking issues (which I suffered from when I tried to implement the above :-))
* initialize firmware pre-fetching when a player connects
* get firmware based on the player's version string as returned by `status.json`
* keep firmware file per platform/branch/resolution combination to support different squeezelite-ESP32 players in an installation
* remove handler to get firmware by numeric ID rather than filename
* only push simple prefs on the list of automatically managed items - hashes and lists can't be managed by the Settings super class
* only push display/artwork related prefs on the list if deal with a player which has a display
* move most artwork/display handling code out of the plugin to the Player class
* rename `eq` pref to `equalizer`, as `eq` is a reserved word
* automatically update the player when the `equalizer` pref is changed
* strip down Plugin to the bare minimum needed with any SqueezeAMP, no matter whether it has a display or not
