Some untranslatable strings

A couple of strings are untranslatable in v0.8.2, please fix this:

• Race/CTS
  • "missing a checkpoint"
• In the HUD editor:
  • “(missing a checkpoint)” in the checkpoint panel
  • the various centerprint example messages
• Minigames:
  • “Well done! Click 'Next Level' to continue”
  • “Game over! Remaining pieces: $PIECES”
  • the names of all those minigames
• Onslaught mode:
  • “Click to select teleport destination” (in xonotic-data: qcsrc/client/hud/panel/radar.qc)
  • “Click to select spawn location” (in xonotic-data: qcsrc/client/hud/panel/radar.qc)
• Gender of model (Multiplayer menu → Profile → written below the model name):
  • “Male”
  • “Female”