Add order sizing based on a fixed amount of risk with preset support

The current function that is supposed to help the user size an order based on a "fixed amount" of "risk" (e.g. $200), taking into account the distance from an already specified entry price and stop loss, really does something else: it provides a way to define the maximum acceptable loss in terms of a percentage of a manually specifiable amount. Also, this amount cannot currently be saved as part of an order preset.

I suggest renaming this feature in an intuitive way and supporting a way to save the fixed amount.

Naming perhaps like this, Risk:
- % of portfolio equity
- % of folder equity
- % of account equity
- % of fixed amount
- % of currency balance
And to additionally provide a true way to size orders based on a fixed amount of risk for a trade with support for saving that amount as a preset. It could be named as:
- fixed amount

There should be no need to enter a percentage in this mode.