Add support for accessing network shares

[hadess]

This works, but the speed is pretty atrocious. I don't know what calls it's doing in the background, but it seems to have problems redrawing fast enough to keep the UI responsive.

[neatdecisions]

Thank you!
Indeed, to have an acceptable UX here we'd need a proper asynchronous processing when switching location.
It's in the pipe, I'll wait with merging this PR until it's done.

[hadess]

The first 2 commits could be merged straight away though.

[hadess]

I've rebased the code, still has the same problems, the first 2 commits could still be merged ;)

One thing I've also noticed is that "Move to Trash" won't work on some remote filesystems, and that the code should check for G_FILE_ATTRIBUTE_ACCESS_CAN_TRASH being true before enabling that feature (and probably also G_FILE_ATTRIBUTE_ACCESS_CAN_DELETE if it doesn't already).