Replace multiplicative alert level overlay with background color change

[GinjaNinja32]

• Removed multiplicative alert-level overlay rect as it makes UI controls much harder to read & use.
• Most ship screens (all except Relay, Strategic Map, and mainscreen/windows) now derive from BaseShipScreen instead of GuiOverlay. BaseShipScreen handles:
  • Background crosses.
  • Background color changing by alert level of current ship.
• Relay & Strategic Map have their alert control upgraded:
  • Red/Yellow alert buttons have different style, so they can be red/yellow (or whatever color the theme says to make them)
  • The main button mirrors the Red/Yellow alert selection buttons' text & style when in red/yellow alert

Images of each station at each alert level, in default theme

AlertLevel::Normal (same as current except that some stations didn't have background crosses)

AlertLevel::YellowAlert

AlertLevel::RedAlert

New Relay/Strategic Map alert control