Stepped trailing strategy example

This is a stepped trailing exit example for educational purpose.

Short brief.
There are 1 stop loss and 3 profit levels.

• When first tp is reached we move stop loss to break-even.
  
• When second tp is reached we move stop loss to first tp.
  
• When third tp is reached we exit by profit.
  

Notes de version:

Stop Loss & Take Profit in %% for independence from instrument's tick size.

Notes de version:

New input `activateTrailingOnThirdStep` added.
With `activateTrailingOnThirdStep = true` when tp2 will be reached, the strategy places trailing stop with amount at tp3 level and offset at tp2 level.
By default the old behavior remains.