main/postmarketos-base-ui-gnome: add showtime to pmb_recommends
So Gnome-based DEs have a video-player by default. The main arguments in favor showtime I see are:
- It's the main candidate to become the new Gnome default video player.
- It's has a mobile-friendly UI.
- It uses HW-decoding and even zero-copy playback by default on many pmOS devices (by using Gstreamer and unlike e.g. ffmpeg-based players like mpv).
- Its main dependencies are shared with other apps like snapshot.
The other candidate that would tick the same boxes but 1. is livi.
This came up when reflashing devices with systemd builds.