x11/xapp: update to 2.2.12

Changes: https://github.com/linuxmint/xapp/compare/2.2.8...2.2.12

Note: port lineage comes from the freebsd-ports-gnome repo since
2018, not what was committed since 2020:

• https://github.com/freebsd/freebsd-ports-gnome/pull/29
• https://github.com/freebsd/freebsd-ports-gnome/pull/33
• https://github.com/freebsd/freebsd-ports-gnome/pull/38
• https://github.com/freebsd/freebsd-ports-gnome/pull/39

Approved by: fluffy (mentor)
Differential Revision: https://reviews.freebsd.org/D35705