Placeholders parameters

[didoda]

This provides a new feature to handle placeholders parameters.

Richtext editors provide already the placeholder utility: you can add BEdita object placeholders inside the rich text.
With this, you will have a list of placeholders after each richtext editor, with the possibility to set "bearing", "pitch" and "zoom" parameters.

Each object type can have its own parameters structure: you can define it using Placeholders configuration. For example:

'Placeholders' => [
    'audio' => [
        'controls' => 'boolean',
        'autoplay' => 'boolean',
    ],
    'files' => [
        'download' => 'boolean',
    ],
    'images' => [
        'width' => ['small', 'medium', 'large'],
        'height' => ['small', 'medium', 'large'],
        'bearing' => 'integer',
        'pitch' => 'integer',
        'zoom' => 'integer',
    ],
    'videos' => [
        'controls' => 'boolean',
        'autoplay' => 'boolean',
    ],
],

Bonus: this introduces EventBus component, that allows developers to handle global events among components.
Usage:

import { EventBus } from 'app/components/event-bus';

// this listen to "my-event-name", lauch callback function when event is triggered
EventBus.listen('my-event-name', callback);

// trigger event "my-event-name" with some data
EventBus.send('my-event-name', data);

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.69%. Comparing base (fda3647) to head (ac01944).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1138   +/-   ##
=========================================
  Coverage     99.68%   99.69%           
- Complexity     1254     1255    +1     
=========================================
  Files            95       95           
  Lines          4836     4851   +15     
=========================================
+ Hits           4821     4836   +15     
  Misses           15       15           

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