feature: group - add reveal-delay config option for drawer

[cebem1nt]

Adds "reveal-delay" option for drawer.

Use case

I have a group module that combines pulseaudio and pulseaudio/slider modules:

    "group/group-volume": {
        "orientation": "horizontal",
        "drawer": {
            "transition-duration": 600,
            "transition-left-to-right": true
        },
        "modules" : [
            "pulseaudio", "pulseaudio/slider"
        ]
    },

The slider will be revealed immediately on hover, but for example, user might have some actions bound to right/left clicking the top module (pulseaudio in this example). There is simply no time to click it. "reveal-delay" adds optional delay in milliseconds before revealing the back module.