Added Powerup Helper powerup, which doubles the amount of time which certain time-based powerups are active, e.g. Plasma Armor duration is doubled from 30 seconds to 60.

The total number of powerups is now 23. Many more are in various stages of completion.

Added Targeting Computer powerup, which replaces the reticle on the HUD pointing to a target with a reticle indicating the direction the player should aim to hit the target (i.e. it takes the target's distance and speed into account). Seeker shots need targeting computer as a precursor.

Targeting HUD tracks mines as well as saucers.

Many texture updates and fixes for minimap and main screen.

Changed some of the availabilities of powerups (first round available and dependencies) to improve game balance.

Enforce condition: Big dumb saucer should always be the first one the player sees.