Add willShow notification to Drop presentation

[ky1vstar]

Hi! This PR adds optional willShow parameter to Drops.show(_:) method to notify user that Drop is about to appear. I.e. it can be used to provide haptic feedback like it is done in SPIndicator, but since Drops are automatically enqueued, now it's impossible to say when you should send such feedback.

[omaralbeik]

Nice one, I was actually thinking of adding this to the Drops class instead, what do you think?

public final class Drops {
    public typealias DropHandler = (Drop) -> Void

    public static var willShowDrop: DropHandler? {
        get { return shared.willShowDrop }
        set { shared.willShowDrop = newValue }
    }

    public static var didShowDrop: DropHandler? {
        get { return Drops.shared.didShowDrop }
        set { shared.didShowDrop = newValue }
    }

    public static var willDismissDrop: DropHandler? {
        get { return shared.willDismissDrop }
        set { shared.willDismissDrop = newValue }
    }

    public static var didDismissDrop: DropHandler? {
        get { return shared.didDismissDrop }
        set { shared.didDismissDrop = newValue }
    }

    // MARK: - Instance

    public var willShowDrop: DropHandler?
    public var didShowDrop: DropHandler?
    public var willDismissDrop: DropHandler?
    public var didDismissDrop: DropHandler?
}

Usage examples:

Show haptic feedback before showing:

Drops.willShowDrop = { _ in
    // Show haptic feedback
}

Show an infinite counter :)

Drops.didDismissDrop = { drop in
    let number = Int(drop.title) ?? 0
    Drops.show(.init(title: "\(number + 1)"))
}