* 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
Sometimes it's necessary to convey more information about the window to screen reader users only (simply putting everything to the window title might be unnecessarily noisy).
For example, Chromium uses that technique to tell screen reader users that the window is in incognito mode (the incognito window looks differently and doesn't have «incognito» in the title, but for blind users the screen reader will announce that it's incognito).