fix(FEC-8079): hide UI on player RESET event

[dan-ziv]

Description of the Changes

Add isStopped state in the engine reducer and do not render the PlayerGUI component when player is stopped! (we still want to render the other components, like VideoPlayer for example, since the video element placed there)

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

[odedhutzler]

so from what i understand here, we will stay with the old ui (and spinner) until the stop flag is turned false (meaning the player reseted?)