Convert to mixin for v1 components

[bedeoverend]

Converts to a mixin for v1 Web Components.

Usage

class SimplaImg extends SimplaElement(HTMLElement) {
    static get simplaConfig() {
      return {
        type: 'Image',
        dataProperties: [ 'src', 'alt' ],
        events: [ 'change' ]
      };
    }
}

Major Changes

• No longer requires Polymer
• Now reads in static simplaConfig property rather than a factory function
• No longer supports "watching" data properties. Now events which should trigger a set back to Simpla need to be declared in the config
• Now requires user to implement the updateFromSimpla and updateSimplaBuffer methods rather than declaring setCallback / getCallback methods in the config, if they want to override the set / get behaviour e.g.

class SimplaImg extends SimplaElement(HTMLElement) {
    static get simplaConfig() {
      return {
        type: 'Image',
        dataProperties: [ 'src', 'alt' ],
        events: [ 'change' ]
      };
    }

    updateSimplaBuffer() {
      if (this.src === DEFAULT_SRC) {
        return null;
      }

      return super.updateSimplaBuffer();
    }
}

Don't care about the names, they were just the private names implemented in the old behavior

Also tweaks how readonly / editable interact with one another, essentally the same behaviour I believe, but a little bit more robust

[bedeoverend]

No idea why tests are failing on FF, works fine locally

[bedeoverend]

Closing in favour of simplajs/simpla#86