Consider using DFileManager

Now that @moochris [got DFilemanager to compile](https://github.com/probonopd/dfilemanager/pull/5) on FreeBSD we can start to consider it for helloSystem.

Looking at the remaining issues:

It seems like DFilemanager is relying on KDE Solid framework and/or UDisks for managing disks.

```
kf.solid.backends.udisks2: Failed enumerating UDisks2 objects: "org.freedesktop.DBus.Error.UnknownObject"
"No such object path '/org/freedesktop/UDisks2/block_devices'"
kf.solid.backends.udisks2: Failed enumerating UDisks2 objects: "org.freedesktop.DBus.Error.UnknownObject"
"No such object path '/org/freedesktop/UDisks2/drives'"
(...)
QDBusConnection: name 'org.freedesktop.UDisks2' had owner '' but we thought it was ':1.12'
(...)
unsupported url QUrl("")
unsupported url QUrl("")
unsupported url QUrl("")
unsupported url QUrl("")
```

Possibly we have the following options, more investigation is needed:
1. __Use the Solid framework__ from KDE (it is in FreeBSD: `kf5-solid KF5 hardware integration and detection`) - what would this involve? (Before we start doing anything: Does it work in KDE Plasma on FreeBSD?)
1. __Use bsdisks__ as a UDisks2 implementation (it is in FreeBSD: `bsdisks UDisks2 service implementation for FreeBSD` - in fact I do see `bsdisks[2170]: "Registering /org/freedesktop/UDisks2/block_devices/da0p4"` in `/var/log/bsdisks.log`) - or would the above already take care of this
1. __Use neither__ (let the FreeBSD automounter handle disk mounting, as we currently are already doing with Filer)

Intuitively, I would be in favor of the last option because
* It works for Filer
* It is most simple
* It probably requires the least amount of new code
* It is the FreeBSD way of doing things
* It does not draw in new dependencies

As for whether we should use DFilemanager instead of Filer, I am thinking about
* Can we make DFilemanager discover (using Zeroconf) and access (e.g., with `ssh://`) network devices? Filer is using gvfs for this, which is drawing in a lot of the Gnome stack and spawns many background processes, so we might want to replace it with something much leaner anyway

What do you think @moochris?