This CL simplifies the keydown handling code and fixes
several issues:

 - Input to the search box was not reliably working, because
   the SVG keydown handler was attached to the window and its
   repeat-key detection was supressing key events.
 - Selecting the input of a node via keys 1-9 did not select the
   input, but always enabled the corresponding input node.
   1-9 now select the input node, and CTRL+1 through CTRL+9 can
   be used to toggle the input edge.

Bug: v8:7327
Notry: true
Change-Id: Ifedc8b703f6552e101ad00fee2f3c50f29b325b5
Reviewed-on: https://chromium-review.googlesource.com/c/1397666
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58579}