Support engine specific hotkey.

Support engine specific hotkey.

BUG=http://crosbug.com/2543
TEST=none

With this CL, each engine can specify one or more special activation hotkeys. This CL doesn't support customizing global hotkeys per engine.
I'd still prefer to customize the global hotkeys based on current locale rather than input method engine.

Add hotkeys property to IBusEngineDesc, so that each engine can specify their own special hotkeys. This is useful for input methods that have dedicated hotkeys, such as Japanese input methods, which has a dedicated hotkey key: Kana.

Review URL: http://codereview.chromium.org/1702015