Add notifications to buds plugin and more

[grazzolini]

• Added notifications when battery drift occurs and when one of the buds is on the case, when they reach same battery level.
• The notifications can be disabled by a configuration option.
• The symbols used to indicate the buds placement can be changed. We use regular characters, but printable unicode characters can be used too.
• Some code was refactored into functions to cleanup run()
• The tests were changed for the symbol change. Also a few tests for the notifications were added. Also, a few tests were extended to make sure the code changing the symbols work.