feat: separate browser notification from ping sound in plugin settings

[GuiLeme]

What does this PR do?

This PR separates the previous pingSoundEnabled which controlled the browser notification as well into the 2 settings:

• pingSoundEnabled: Same name, different feature - It controls only the ping sound;
• browserNotificationEnabled: Now, this flag controls the browser notification;

One other important change is: the ping sound comes enabled by default while the browser notification doesn't.

Motivation

For the browser notification to appear, it needs to ask for permission from the browser, which is not the ideal for every user - it can cause the person to question their privacy while using BBB. Despite that inconvenience, it's good to maintain the ping sound.

A good solution, then, is to separate the flag to enable the ping sound from the flag to enable the browser notification.

More

This PR depends on the recent refactor from #68.

[ramonlsouza]

if nothing is added to BBB's settings.yml file (default settings), an error is displayed and the ping sound is not played

[GuiLeme]

@ramonlsouza I fixed that issue and found another one: Sometimes, if the current user is selected, the ping sound plays multiple times - I fixed that too in the same commit.