Use Fixed Location for KIO FUSE

The path of the KIO FUSE mount changes between launches (and in particular, between sessions) meaning that programs that have an "Open Recently" feature, don't work as expected. Bug Report: 432856.

Mount KIO FUSE at /run/1000/kio-fuse/ instead of /run/1000/kio-fuse-<6-char-random-str>/. The reason why there's a random string is so that if KIO FUSE crashes for whatever reason, it can be DBus activated again. Otherwise, if KIO FUSE crashes and it is DBus activated again, it will be unable to mount as the previous process crashed and didn't unmount itself. I don't know if there's a way to call fusermount3 -u $dir upon crashing (which would allow us to remove the random string). The following two ML threads (here and here) lead me to believe that this is possible if mounting with -o auto_unmount but my local testing seemed to imply it didn't work. This will need investigating.
For the subsequent stat to succeed, the origin (i.e. protocol + authority) must already be mounted somehow. I'm not sure on the best way to proceed on that though... although it's definitely possible one way or another.