Design listeners management mechanism/approach.

[SerhiiYahdzhyiev]

The main focus should be kept on:

• Providing convenient ways for the end users for adding and removing user defined listeners
• Avoiding potential conflicts between predefined and user defined listeners.
• Providing a way of "decorating" internal listeners.

Important to consider:

• Number of predefined listeners should stay rational and manageable.
• User defined listeners should not have direct access to modify and internal data structures of the library.
• All user interactions with listeners management mechanisms should be provided via TabsManager instance methods.