Keyboard shortcuts

[Leemoonsoo]

This PR implements issue #216

• Implement shortcuts
• Let paragraph be focused even if code is hide
• Add shortcuts to help menu

[swkimme]

Cool but what if we also insert VI style shortcuts?
like,
shift-ctrl-j => move down
shift-ctrl-k => move up
shift-ctrl-d => remove paragraph

[swkimme]

@Leemoonsoo may I update this PR with VI style shortcuts?

[swkimme]

TODO: show paragraph focus status

[Leemoonsoo]

@swkimme Yes, sure.

FYI, current implementation of key event handling is done by code editor.
I realized limitation of this approach, that's why i stopped working on it.

for example, shortcut does not work just after i change chart type. (editor lose focus)