Eliminate GUI library dependencies for `nextcloudcmd`

### Expected behaviour
On a headerless machine, nextcloud-desktop-cmd should not require X11.

### Actual behaviour
I cannot purge about 315 MB of X11 libraries because nextcloud-desktop-cmd requires them.

### Steps to reproduce
1. Install package nextcloud-desktop-cmd on debian 9.9 (stretch)
2. Attempt to purge X11 libraries. E.g.:

for i in $( pre x11 | cut -f1 ) ; do echo $i ; apt purge $i ; done

nextcloud-desktop-cmd will be either in the list of dependencies to be removed or in the list of packages to be removed with "apt autoremove".

3.

### Client configuration
Client version:

root@dzur:/crc/back/white.09# pre nextcl
libnextcloudsync0 2.5.1-1~bpo9+1 amd64
nextcloud-desktop-cmd 2.5.1-1~bpo9+1 amd64
nextcloud-desktop-l10n 2.5.1-1~bpo9+1 all
root@dzur:/crc/back/white.09#

Operating system:

debian 9.9 (stretch)

OS language:

English

Qt version used by client package (Linux only, see also Settings dialog):

5.7.1+dfsg-3+deb9u1

Client package (From Nextcloud or distro) (Linux only):

root@dzur:/crc/back/white.09# pre nextcl
libnextcloudsync0 2.5.1-1~bpo9+1 amd64
nextcloud-desktop-cmd 2.5.1-1~bpo9+1 amd64
nextcloud-desktop-l10n 2.5.1-1~bpo9+1 all
root@dzur:/crc/back/white.09#

Installation path of client:

/usr/bin/nextcloudcmd

### Server configuration

Nextcloud version: Nextcloud 15.0.9

Storage backend (external storage):

### Logs

Please use Gist (https://gist.github.com/) or a similar code paster for longer
logs.

1. Client logfile: Output of `nextcloud --logwindow` or `nextcloud --logfile log.txt`
(On Windows using `cmd.exe`, you might need to first `cd` into the Nextcloud directory)
(See also https://docs.nextcloud.com/desktop/2.3/troubleshooting.html#log-files)

2. Web server error log:

3. Server logfile: nextcloud log (data/nextcloud.log):