Merge branch 'mainloop-cleanup' into 'master'

Mainloop cleanup

See merge request GNOME/gtk!1404

diff --cc docs/reference/gtk/gtk4-sections.txt
Simple merge

diff --cc gtk/gtkmain.c
Simple merge

diff --cc tests/testclipboard2.c
Simple merge

diff --cc tests/testdnd2.c
Simple merge

diff --cc tests/testicontheme.c
Simple merge