Add full support of changing merc card and its abilities

[piedpiper358]

[sebastientromp]

Isn't there another event than GAME_STATE_UPDATE that could be used for that? That event is a crutch to give regular sync points between the logs and the app, but ideally I would like to get rid of it.
It also triggers quite frequently, so hooking events to it will lead to a lot of unnecessary computation

[piedpiper358]

I tried to avoid using this event (see #279), but it is seems impossible to get complete information from other events.

[piedpiper358]

We can handle merc transformation into an alternative form (CARD_CHANGED_ON_BOARD), as well as abilities that change on activation (MERCENARIES_ABILITY_ACTIVATED) but there are abilities that change when certain conditions are met (Drek'thar, Elise, Blink Fox) and their change can only be processed with GAME_STATE_UPDATE.

[piedpiper358]

I double-checked it: we can use TURN_START event instead. This will work for everything except Blink Fox's Mind Thief. It is updated after start of turn.

[piedpiper358]

@sebastientromp, I changed handled event to TURN_START, so now it will be called only once per turn. (Blink Fox's Mind Thief ability change is also handled thanks to a small change to mercenaries-speed-parser)

[sebastientromp]

Thanks. I will still need to take some time to investigate this on my side, as I would rather use dedicated events that describe what is actually happening, instead of relying on existing event side-effects.