mac: Implement global keyboard shortcut API.
win: Implement global keyboard shortcut API.
linux: Implement global keyboard shortcut API.
Emit failed message when fail to register shortcut.
:memo: Shortcut API document.
Nits: Fix code style.
Update GlobalShortcut API design.

* Rename Shortcut API to GlobalShortcut for better suite.
* Implement the new design interfaces.
* Put the chromium related source code to the same directory as in chrome.
Merge pull request #534 from hokein/hotkey

Implement global shortcut API, fixes #439