UI: add "stop markers" to game panel

Sometime player want to stop on special step like opponent's "declare attackers" step to cast spell. Current version uses one mechanics:
* stops on every step, skips by skip buttons;
* you can setup stop steps in preferences, but it's useless -- you can't change it in the middle of the game;
* it's annoying -- you must clicks on every steps/priority until needed position (and it's easy to miss that by wrong clicks).

Special "stop markers" panel can help with that. That's panels uses many games like MTG Arena and other CCG:

That's why xmage can get same panel:
* loads stop panel with user's settings from preferences on startup;
* users can enable/disable stop markers by click on it;
* users can reset stop markers to default (from preferences);
* skip buttons have priority over stop markers;
* shows current step on that panel (by arrow or colorizing);

Example: