feat(power): Support power button and power LED

[XavierBrassoud]

Implements 3 new methods:

/**
* @brief Determine if the power button is pressed or not. The power button has a dedicated pin.
*
* @return true
* @return false
*/
bool isPowerButtonPressed();

/**
* @brief Control the state of the power led (active LOW).
* Depends on synchronize().
* 
* @param state LOW: on / HIGH: off
*/
void writePowerLed(uint8_t state);

/**
* @brief Get the state of the power led (active LOW).
*
* @return uint8_t LOW: on
* @return uint8_t HIGH: off
*/
uint8_t readPowerLed();

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Synchronize states through synchronize():

/**
* @brief Read and write to shift registers that control buttons, power led and display.
* Called each time a refresh is needed.
*/
void synchronize();

Behavior of readButons() changes due to new synchronize() method. It's mandatory to call synchronize() prior to readButons():

controlPanel.synchronize();
buttons = controlPanel.readButons();

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Examples OneButton updated and blink added.