Use symbolic enum values in the Loopy presets array.

Use symbolic enum values in the Loopy presets array.

The use of plain numbers was needlessly confusing, and in particular
made it too easy to make unintended changes to the existing Loopy
presets when inserting a new grid enum value anywhere other than at
the end of the list.

But in the course of doing this I realised that, against all
sensibleness, the numeric indices for grid types in grid.h and in
Loopy itself don't match up! Right now I don't want to get sidetracked
into fixing the structural confusion that made that happen in the
first place, but I've at least materialised Loopy's version of the
enum with clearly identifiable LOOPY_GRID_* names.