Warn users not to run as root

[LeafHacker]

Is your feature request related to a problem? Please describe.
Some users have been running the Installer as root (e.g. using sudo). This is not only a potential security issue for them, but it also breaks the .minecraft directory locating process, as it looks in root's home directory not the user's.

Describe the solution you'd like
Show a warning message when running as root on UNIX based operating systems.

Describe alternatives you've considered
Block running as root entirely (this is probably overkill, there may be a genuinely useful reason to run as root).

We could also show a .minecraft selector in the GUI if running as root?

Additional context