MD Bookmarks: Improve keyboard shortcut targeting

Keyboard shortcuts are no longer triggered by a global event listener
looking at all events on the page. Instead, we process keyboard
shortcuts as part of the existing input processing for the tree and the
list, with an additional listener to pick up any key presses while
nothing is focused.

This has two benefits:

1. Pressing a keyboard shortcut with a sidebar folder focused will
   process shortcut for that node, rather than for the list selection.
2. Keyboard shortcuts will no longer activate when dialogs/buttons/input
   fields are active

BUG=722300
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2885353002
Cr-Commit-Position: refs/heads/master@{#477468}
9 files changed