Make drag-selection the default behaviour, allowing (almost) any part of
the cell to initiate a select.
a small 5x5 rect at the corners of a cell can be used to initiate a
drag-copy instead.
Fixes#4268.
To initiate drag-to-select, the user can move the mouse to near the edge
of a cell, and click-and-drag when the cursor changes to a crosshair.
Fixes#4167.
Every time the scrollbar reaches the end, we append 100 more rows
(seamlessly!).
As a result of defaulting to 100 rows, we can also save with the
smallest number of rows required.
This partially deals with #4170.
Customize the cell editing delegate to stop editing when one of the
various cursor movement keys is hit. This allows you to type into a
cell and then move to an adjacent cell by simply pressing an arrow.
This may not be the best factoring for this feature, but it's pretty
dang cool and we'll see how it evolves over time. :^)
