Implement placeholder text for stickers

Related to #14; as requested fourteen minutes ago on the forum at https://forum.sailfishos.org/t/whisperfish-the-unofficial-sailfishos-signal-client/3337/862?u=rubdos

added enhancement label

enabled automatic add to merge train when the pipeline for 77d92320 succeeds

added MR statusready label

started a merge train

mentioned in commit 94bc09cd

merged