Pulls the actual parsing of a hotkey into an icon out into its own function

Modifies engines\default\engine\HotkeysIconDisplay.lua to better support addons. The current call to display() draws every hotkey over the course of a nested iteration, which means that if someone wishes to modify this behavior (e.g. have flagged talents use a different border color, or support multiple talent icons to denote different states of a talent), the entire display() function must be entirely replaced. This isn't ideal as these modifications are mutually exclusive.

This MR splits the actual parsing of a hotkey into a displayHotkey() function (with appropriate arguments) so that addon developers can safely superload the hotkey drawing behavior.

This MR is save-safe.