grafx - Extensible Java Graphics Package Code

Adding enough requestUpdate() calls so that a blank screen would not happen
with JVMs that have some weaknesses. I really don't like this way of doing
things, yet I don't seem to have control over the quality of the now-Oracle
controlled JVMs. The audio modules are also quite stubborn and buggy.

In levels 21 & 22, changing height of pit (z value) from 10 to 11. More space
in case the block sequence is unfavorable by chance, so there's more room to
recover.

Adding 'very-tricky.blo'.

Fine-tuning level 21.

Adding 'excellent4.blo'.

Minor adjustments on some of the new levels.

Adding 'excellent3.blo'.

Swapping levels 21 and 22, based on difficulty.

Adding a new level 22.

Adding a new level 24.