Commands - implementation of `Listener` & command procedures

[BartoszKlonowski]

The goal of this issue is to implement the full Listener feature.

The implementation should apply to both Popup script and content script. Both layers should be able to communicate effectively using commands and controls.

It should be able to:

• Listen for clicks of each button and recognize them separately
• Listen for session name change in the <input> form
• Push the commands to the content script for further processing
• Recognize each click with separate message

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The code should be covered with unit tests.