* fix: i18n of gtk msgbox buttons
similar to #19756 (12df0e8) but for messageboxes
* refactor: DRY the gtk+ button mnemonics
* fix: don't compile gtk_util on non-Linux platforms
rename from `gtk_util.[cc,h]` to `util_gtk.[cc,h]` so that it gets
picked up by the `extra_source_filters` rule in `BUILD.gn`.
* fix: make linter happy
It really shows that I cannot build locally atm... :P
