Major changes in 1.15.4
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* gdu,udisks2: Make use of new ID_FS_BOOT_SYSTEM_ID udev property
* fuse: Install systemd tmpfiles.d exclusion file
* mtp: Fix crash when libmtp operations fails without error message
* mtp: Respect remove_source when pulling a file
* mtp: Respect remove_source in do_push
* tests: Add gvfs-testbed to enable tests which need root
* tests: Check for URI string presence on CLI unmount
* Translation updates