Synchronize the editor content after the editor is ready

[psmyrek]

Suggested merge commit message (convention)

Fix: Synchronize the editor content after the editor is ready. Closes #21.

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Additional information

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[psmyrek]

The easiest way to reproduce this error is by delaying the editor initialization:

1. Go to master branch.
2. In the mounted() hook, add a timeout before setting up the internal component data:

   this.editor.create( this.$el, editorConfig )
   +   .then( editor => new Promise( resolve => setTimeout( () => resolve( editor ), 5000 ) ) )
       .then( editor => {

3. Build the editor: npm run build.
4. Open the test sample. If you change the model value in the textarea (below the component) within 5 seconds, warnings and error will be logged in the console.
5. Then check if the same scenario does not throw any errors and warnings on the i/21 branch. Remember to re-build the editor. The model value in textarea and in the editor content should stay in sync.

[pomek]

LGTM.