Add persistent user markers

[Warhammer47]

I think it would be very interesting to implement the possibility to place permanent markers in the map (savable but can be deleted). This could be useful when:

• Marking enemy checkpoints.
• Specify mine positions in mine fields (friendly and enemy mines).
• Tactically important observation posts or other crucial locations. This could provide a more friendly, realistic and sharable aspect (especially in dedicated servers)

[LordGolias]

I like it. This should not be difficult to do.