Add new toast UI and operation hint message

[XuYicong]

Partly implemented #74

new_toast.mov

5 seconds after launch, if keymapping is not enabled, a message would appear prompting the user to enable keymapping. In this case, when the user enables keymapping, the previous message disappears and a message would appear prompting the user to disable keymapping. In this case, when the user disables keymapping, the message would disappear. Both messages show at most once during each launch.

Keymapping editor open and close messages are also migrated to the new UI. The open message would stay there until the user closes the editor. The close message would stay for 3 seconds.

[IsaacMarovitz]

A couple of suggestions, capitalise Option, and secondly, set a max number of toasts. The video shows about ~10 onscreen at once, but we could probably limit this to like 4, i.e. quickly destroy any toasts at the end of the list if the limit is going to be exceeded. Either that or iOS notification style have them stack up if there's too many (might be kinda difficult tho don't wanna make ur life hard :kek:)

[FlyMeToTheMoonAndLetMePlayAmongTheStars]

I think option is fine as it is, it's more consistent with the actual hardware key since that's not capitalized either. The color contrast already makes it more prominent/distinct.

[XuYicong]

I agree that small option is more consistent with what is written on the hardware keyboard. And I limited number of toasts to 4.

limit4.mov