Level: make combo counter appear at combo 2 in main menu.

Alas, this makes this well-hidden feature a lot less easter-egg-ish,
but getting a 10 hit combo was nearly impossible anyway.
Level: decouple blob spawning from item updating.

Also slows down the rate at which blobs spawn from once every 7 frames
to once every 10 frames.
Level: make main menu hover color changes not look like garbage.

Previously, only one item would be updated per tick (or menu loop,
whatever it is that controls the loop speed here), which made the
fading in/out animation very chunky.

Now all of them are updated each tick, which makes the animation far
more smooth.

A new constant has been introduced, kTitleDefaultGlow, which specifies
the default darkness of the title items.