This plugin currently uploads the selected sticker every time, to the
HTTP File Upload service of the server (see XEP-0363), a future
optimisation would be to use XEP-0231 instead, for better caching on the
recipient side.
It relies on a helper tool to select the wanted sticker inside the pack,
a sample one is provided in tools/sticker-picker/, but it is not built
by default.
