* refactor: convert Menu and globalShortcut to gin
* refactor: convert api::Cookies to gin
* refactor: convert View and WebContentsView to gin
* refactor: convert WebContents related classes to gin
* refactor: convert powerMonitor to gin
* refactor: prepare for header change
* refactor: remove last uses of mate::EventEmitter
* refactor: remove mate::EventEmitter
* refactor: move trackable_object to gin_helper
* fix: custom converter should not use Handle
* fix: no more need to check if icon is empty
It was a bug that the Handle<NativeImage> can be non-empty when the
image file does not exist. The bug was caused by the converter code
writing out the image even when the convertion fails.
The bug was work-arounded by adding an additional check, but since the
original bug had been fixed, the additional check is no longer needed.
* fix: should always set frameId even when callback is null
* fix: do not mix gin/mate handles for NativeImage
