Named levels instead of PAGE and LEVEL number. But...

Lots of code still depends on PAGE and LEVEL variables. They used
to be part of BlocklyApps, but are now part of Turtle, so that I
can make progress further simplifying the app structure. More data
is parameterized via the levels.js file: namely startBlocks and toolbox.
Both still use page and level numbers in their templates, but those
numbers do not need to correspond with the level name at all anymore.

Several places where MAX_LEVEL was checked, now checks
Turtle.level.freePlay directly; was broken with out that change.

answers.js no longer modifies the Turtle global, but still needs
Turtle in scope for executing answers. Answer proceedures are not
yet part of the level data structure (must use PAGE and LEVEL).

level.instructions parameter is now accepted from main