graphics/gtk-update-icon-cache: use triggers

Use triggers instead of enforcing the INSTALL_ICONS which enforces
gtk-update-icon-cache dependency on every ports installing icons.

The bonus:

• factorize gtk-update-icon-cache update at the end of the build
• stop adding a gratuicious dependency on gtk-update-icon-cache on packages which do not need it
• works on packages where INSTALLS_ICONS were forgotten

PR: 258502
Exp-run: antoine
Reviewed by: tcberner
Differential Revision: https://reviews.freebsd.org/D31960