actions-popover: Save screenshots into ~/Pictures/Screenshots
Ever since GNOME Shell 42, the screenshots are now saved under XDG_PICTURES_DIR/Screenshots.
Boxes will attempt to create the directory if it doesn't exist.
This directory name is translatable and both GNOME Boxes and GNOME Shell translations for the directory name should be the same for the same folder to be used.
Fixes #776 (closed)