patch: initialization of textStyle object

[dvh91]

Description of the Changes

textStyle object initialized so the UI could understand what the initial textStyle is and show accordingly the captions style options.

CheckLists

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• test are passing in local environment
• Travis tests are passing (or test results are not worse than on master branch :))
• Docs have been updated

[dan-ziv]

Move initialization to constructor

[OrenMe]

@dvh91 don't know if you deleted the default Pull Request template or it didn't appear but we have it there for a reason so please add it to the first comment of this PR(just edit it) and add details and tick the corresponding check boxes after you save it.
Thanks!

[OrenMe]

@dan-ziv ES6 classes member initialization can be done like this in general, and it is exactly the same as constructor initialization AFAIK.
Only issue is the fact we do some in constructor and some in member definition, so there's no alignment.
By the way, if you check the actual transpiled class you will see it does the _textTrack init in the class function definition, e.g.

function Player() {
   ...
   this._textTrack = new ...
   ...
}

[dvh91]

@OrenMe - Updated the first comment with the pull request template.

[dan-ziv]

@OrenMe
I know that,
The reason I asked this is only for consistency.