Trash support

[garrett]

There's an XDG specification for deleting files by moving them to the trash:

https://specifications.freedesktop.org/trash-spec/trashspec-latest.html

Having a trash would also help with undo (#519) for files that have been "deleted", as long as they're not deleted permanently.

Requirements:

1. Move file to trash location ($XDG_DATA_HOME/Trash/files (home files), $topdir/.Trash/$uid/files (device), or $topdir/.Trash-$uid/files (device fallback) — see spec) and write a metadata file (with original path and time of trashing) in ../info (relative to the files directory)
2. Provide a way to undo, such as menu entry, keyboard shortcut, and possibly a toast (see Undo support #519)
3. We'd also need a global action to "Empty trash"
4. Possibly also a way to view the trash? (Nautilus has this and it's unified across home and devices. In other words, it's viewed as one trash. It also has a contextual banner to empty the trash when viewing it.)