DevTools: introduce editor shortcuts section in shortcuts screen

TL;DR: This patch reorganizes shortcut listing on Shortcuts Screen in
devtools and adds 4 more editor shortcuts.

So far we've used to have a single "Sources panel" section in shortcuts
screen. However, with the addition of 4 new editor-specific shortcuts
into the listing (Ctrl-M, Ctrl-D, Ctrl-U, Ctrl-Space) the list becomes
way too hard to read.
The patch suggests splitting the section into "Debugger" and
"Editor".

R=vsevik, pfeldman

Review URL: https://codereview.chromium.org/390113002

git-svn-id: svn://svn.chromium.org/blink/trunk@178174 bbb929c8-8fbe-4397-9dbb-9b2b20218538