Remove hard dependency on systemd (to support systems like Slackware)

[TronNerd82]

Pre-submit checks

• [X] I checked for similar issues beforehand, but could not find any. I could not add my proposal to any existing issue.
• [X] I am going to take the time to to fill in all the required details. I know that the bug report will be dismissed otherwise.

Feature description

I attempted to install AppImageLauncher on Slackware, but to no avail.

Slackware is perfectly capable of running AppImages, as is just about any modern distro, but when I tried installing AppImageLauncher using the lite AppImage, it failed to install, throwing up an error related to systemctl not being present.

Slackware doesn't make use of Systemd or any other common init system (which I'm assuming AppImageLauncher relies on), instead preferring SysVinit with BSD-style init scripts.

Proposed solution

If I could, I would help, but I am nowhere near competent at programming actual stuff yet. I would like to see support for Slackware come along in the future, but I don't expect it to happen anytime soon. I'm pretty patient, however, so immediate action is not imperative.

Alternative solutions you considered

No response

Additional context

Please forgive me if this issue seems rather unprofessional and poorly written, this is my first time.