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
Changed Hub.registration_timeout to be a config input.
remove raw cell placeholder on focus, closes #5238
update notebook quickhelp on new shortcuts
removing keyboard shortcuts should also update help
rebuild.QuickHelp event sets the dirty bit

QuickHelp is rebuilt on show if the dirty bit was set.
remove recreation of QuickHelp
Workaround for Tooltip & completer click focus bug.
Remove all should cancel blur logic.
Add illusion that cell is in edit mode when complete is up,
even though it is not in focus so it is not in edit mode.
first pass at capitalizing keyboard shortcuts
prettify combo keyboard shortcuts