v0.4.0b5

Beta.

• New: Channel X delete preset on every channel. It deletes the preset that
  channel's select is showing — one row above the button — so no name has to be
  retyped. It is greyed out while the channel is not set to a preset.
• Presets stay charger-wide: deleting from one channel removes it everywhere,
  as skyrc.delete_preset always did.

Unchanged from b4: Channel X preset name plus Channel X save preset,
per channel; storage runs report charging/discharging; the options and reauth
dialogs load; entered values survive program changes and disconnects; and the
config-side entities load with the charger out of Bluetooth range.