Replace confirm() with custom pop-up when starting or deleting a game

[Valtteri Heikkilä]

A custom pop-up would be better than the modal confirm() used currently (0.3.1-alpha).

Pros:

• No stuttering as pop-up doesn't block audio updates unlike confirm() (see also #3)
• Different visuals can be used to clearly distinguish between start and delete
• Hence second confirmation on delete can be removed
• Pop-up could be also used to report messages to the user (relating to #12)

Cons:

• Some added complexity in the front-end

[Valtteri Heikkilä]

Removed most of confirm() dialogs. Only one at deleting a game remains.