New gradient effect on textbox background

Most of the Specs game UI has subtle gradients, which made the flat
black textboxes look out of place. I should probably update the
MenuStrips too...
Add more DBG_DISABLE_x killswitches

Being able to suppress battles may come in handy later...
Implement Insta-Kill attack effect

instaKill effect does damage equal to the victim's maxHP, which means it
can be defended against once I get around to implementing the Defend
command...