sticker selector

[cyBerta]

As a follow-up for #1661, we can reuse much of the logic and UI to implement a sticker selector.

For now we will add another entry to the clipper icon menu. Tapping on the entry will open a ViewController with a grid, similarly to the current image selector or gallery view.

For the beginning only images from messages with the message type DC_MSG_STICKER should appear in the grid.

Tapping on a grid tile should send the sticker immediately, instead of adding it to the draft area.

[Simon-Laux]

does that mean we can paste images without them getting converted to stickers eventually?

[r10s]

the sticker selector is not about the paste behavior which was introduced to support stickers from the system keyboard. doing that right seemed to be a larger impact that time, however, of course, maybe this can be improved as well and stickers can be detected more on point, but this is another thing.