main/postmarketos-base-ui-gnome: add showtime to pmb_recommends (MR 6038)

So Gnome-based DEs have a video-player by default. The main arguments
in favor showtime I see are:
1. It's the main candidate to become the new Gnome default video player.
2. It's has a mobile-friendly UI.
3. 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).
4. Its main dependencies are shared with other apps like snapshot.

The other candidate that would tick the same boxes but 1. is livi.
[ci:skip-build]: already built successfully in CI

main/postmarketos-base-ui-gnome/APKBUILD

 # Maintainer: Oliver Smith <ollieparanoid@postmarketos.org>
 pkgname=postmarketos-base-ui-gnome
 pkgver=4
-pkgrel=3
+pkgrel=4
 pkgdesc="Meta package for shared configuration for GNOME-based UIs"
 url="https://postmarketos.org"
 arch="noarch"
@@ -50,6 +50,7 @@ _pmb_recommends="
 	loupe
 	nautilus
 	papers
+	showtime
 	snapshot
 "