gtkplacessidebar: Add loop and device-mapper devices

Loop and device-mapper devices were not included in the places sidebar,
but those are often interesting to the user, for example mapped
encrypted volumes stored in the filesystem.
1 job for loop-and-dm-devices-in-sidebar in 7 minutes and 54 seconds (queued for 7 minutes and 7 seconds)
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passed #53731
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00:07:54