start better keyboard handling
refactor to improve cell switching in edit mode

This code was repeated in both CodeCell and TextCell, both of which are
extensions of Cell, so this just unifies the logic in Cell.

TextCell had logic here to check if the cell was rendered or not, but I
don't believe it is possible to end up triggering such a code path.
(Should that be required, I can always just add back these methods to
TextCell, performing the .rendered==True check, and calling the Cell

prior to this, code mirror at_top would only return true on if the
cursor was at the first character of the top line. Now, pressing up
arrow on any character on the top line will take you to the cell above.

The same applies for the bottom line. Pressing down arrow would only go
to the next cell if the cursor was at a location *after* the last
character (something that is only possible to achieve in vim mode if the
last line is empty, for example). Now, down arrow on any character of
the last line will go to the next cell.
DRY: factor out common handle_keyevent method

This method was identical in both CodeCell and TextCell
typo, use keycodes object instead of magic numbers
DRY: refactor common keyboard handling to Cell

Our edit mode keyboard shortcuts don't distinguish between being in a
code cell or in a text cell, so it makes sense to handle both in one
place. This is a first step in that direction.
added new use_shortcut method to shortcuts

this way, you can ask if a particular event will be handled by the
shortcuts system. This takes away the need to special-case many
different possible keys which should be ignored by codemirror by
ignoring them en masse.
up and down keys no longer need special casing
no need to special-case Escape anymore
dem semicolons
more semicolons
minor fixes to docs to address PR feedback
it's only half true that I'm anal about semicolons
another doc fix
renamed 'use_shortcut' method to 'handles'
go to appropriate line when coming from another cell

Sets the cursor on the last line of the cell when moved up from the top
of the cell below, and sets the cursors to the first line when moving
down from the bottom of a last line.

Here, we retain the character that the cursor was on, so that users
wishing to have up-down functionality like one document can still use
this shortcut handler and simple adjust the at_top and at_bottom methods
restore master behavior

up arrow at the top line first goes to char 0, and only goes to the cell
above if already on char 0. Same with down arrow on the bottom line:
transition cursor to the end of the line, and only go down a cell if
already at the end of the last line.

this makes for an unhappy experience in code-mirror's vim mode  for j
and k keys, but we'll fix that in the next commit
go to the beginning of the line when entering cell
fix up and down arrows in the top and bottom cells
brief key event desription
Merge pull request #5296 from ivanov/more-shortcuts

unify keyboard shortcut and codemirror interaction