Refactor cursor movement

[mjakeman]

Cursor movement is currently split between editor.c and display.c which is very undesirable. Movement should be handled at a single location, preferably at a level just above layout - it is layout dependent.

The cursor and selection should be separated from the general idea of 'marks' (i.e. cursor/selection is a mark, but not all marks are cursors). We'll also want some way to index text without using marks, in the case of temporary or read only access to the document model.