Backport !3113 “gaction: Validate actions activated over D-Bus” to glib-2-74

This is a non-trivial backport of !3113 (merged) to glib-2-74 in order to fix #1904 (closed).

It includes 5 patches from !3073 (merged) needed to prepare the code for the 9 commits in !3113 (merged) and avoid conflicts.

Despite that, there was still a change required to “gactiongroupexporter: Validate actions activated or changed over D-Bus” to add some additional braces to prevent a -Wdeclaration-after-statement warning: main has disabled that warning, but it’s still active on glib-2-74, so we need to comply with it in the backport.

Other than that the patches are unchanged from main and all unit tests pass for me.

Signed-off-by: Philip Withnall pwithnall@endlessos.org