Added kill boss tutorial overlay.

Removed boss hint arrow.

Removed Game UI from boss level to make it feel different.
This included removing the 'DISTANCE LEFT' meter which players found
confusing.
Tweak to level 1-6.

Some small UI tweaks to enemy dialog box.
Fixed restore without save bug.
Some more level tweaks.
Made death ray easier to kill but worth less points.

Fourth boss is now lighter.

Changed 4-6 to be easier to navigate, since the emphasis should not be on
super-fine mechanics.