Add Event to Indicate Transmission Has Occurred

[bpe2]

Add a callback & corresponding event to the NodeConfiguration class to indicate the Node has begun to transmit.

This event controls the display of a "bubble" in the application. Potentially with a size/growth rate configurable in the module. Along with potential other indicators.

Potentially configurable items of the "bubble":

• Target size
• Display duration
• Growth rate
• Color
• (Maybe More?)

See also: #26