Fix missing icons + script

[mhoangvslev]

I feel like $HOME is enough, instead of cat /etc/passwd | grep -w $current_user. I also added the missing icon in .desktop file and script generation

[rippin93]

Thanks for adding the icon, my display manager (i3) does not show any icons attached to .desktop files, so this fell under the radar here.

For the removal of $HOME, have a look in #2, there were many problems with this particular variable which is why I removed it from the build process completely (seems like some shell implementations seem to not set $HOME as a default variable).
I really don't want to remove this piece of code since I can't cover every shell environment which uses anything outside of any linux base tools (and whoami is used in the linux kernel itself which is why I used it to cover all derivates).

As for the .desktop file generation: I do not really like the approach of creating the .desktop file contents inside of an installer over having a ready-to-ship file in which only a placeholder needs to be replaced.