new_grid: Rewrite of Snap and Grid. (squashed commit)

new_grid:  Rewrite of Snap and Grid. (squashed commit)
Separate Snap from Grid.  Lots of naming changes.
Multiple simultaneous snap options allowed. Grid is one of the possible Snap options.
Grid uses the same data as the rulers.  Replace complicated tempo_lines with simple grid_lines.
The Grid is zoom-scale-sensitive along with the rulers.  If you are zoomed out, grid becomes coarser.