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
remove raw cell placeholder on focus, closes #5238
semantic names for indicator icons

For all of the discussion that we had about what kind of icons should
and should not be used to indicate what mode the notebook is in, we
never went through to make it possible to override it.

With this change, it is now possible to override what icons are
displayed for Command and Edit Modes.

For example, @minrk liked the fighter-jet icon for Command Mode, so he
can put this in his custom.css

.ipython-command-mode:before {
     content: "\f0fb";
}
semantic names for kernel status indicator
ensure ipython-command-mode icon displayed on load
better fix for starting with Command Mode icon
give hint on where to find custom.js
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'