Add indication for duplicate stickers

[Bheniamyn]

When adding a sticker to a pack display a visual indicator at the respective pack to show the user that the sticker already exists in this pack. Alternatively the pack can be grayed out to show that.

[gthvmt]

Good idea, I'd prefer the option of graying out stickerpacks which already contain the given sticker. I am already graying out stickerpacks which contain animated stickers when adding a static sticker and vice versa. Maybe it would even be helpful to display a dialog when the user is tapping a grayed out stickerpack which informs him why it is disabled in that specific case 🤔