Add log level feature

[epiclabsDASH]

Improves the logging system so it works with log levels (as requested in #2601).

A couple of methods have been created in Debug class to allow developers to set/retrieve the log level:

• Debug.setLogLevel
• Debug.getLogLevel

The following log levels are supported:

• dashjs.Debug.LOG_LEVEL_DEBUG: what we have today. Everything.
• dashjs.Debug.LOG_LEVEL_INFO: key events (ex: segment loaded, bitrate changes).
• dashjs.Debug.LOG_LEVEL_WARNING: (ex: timecode jumping, fetch is not available, EME not available, adaptations set that are not supported, semantic problems with the mpd that dont avoid playback to start).
• dashjs.Debug.LOG_LEVEL_ERROR: errors that don't stop playback (ex: segment 404, codec errors - fallback).
• dashjs.Debug.LOG_LEVEL_FATAL: Playback fails completely (codecs not supported).

With the new approach, any dash.js class that wants to use the new Logger has to create its own Logger instance. Example:

logger = Debug(context).getInstance().getLogger(instance);
logger.warn('This is a warning message');

Additionally:

• Method Debug.setLogToBrowserConsole has been deprecated and will be removed two versions after current one (v2.7.0)
• Method Debug.getLogToBrowserConsole has been deprecated and will be removed two versions after current one (v2.7.0)
• Event Events.LOG has been deprecated and will be removed two versions after current one (v2.7.0)

Documentation, definition file and unit tests updated.