[OSX] Add iconsAreTemplates property to Tray API objects

Add boolean property (defaults to `true`) `iconsAreTemplates` to `Tray` objects
to allow for proper display of icons in Mac OS X (Yosemite) Dark Menus.

When `iconsAreTemplates` is set to true, both `icon` and `altIcon` are treated
as "templates" and the system automatically ensures proper styling according to
the various states of the status item (e.g. dark menu, light menu, etc.).
Template images should consist only of black and clear colours and can use the
alpha channel in the image to adjust the opacity of black content.

See [Dark Menus in AppKit Release Notes for OS X v10.10](https://developer.apple.com/library/mac/releasenotes/AppKit/RN-AppKit/#10_10DarkMenus)
and [`NSImage setTemplate:`](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSImage_Class/index.html#//apple_ref/occ/instm/NSImage/setTemplate:).

On Linux and Windows setting the property has no effect.

FIX rogerwang/node-webkit#2476
[OSX] Add iconIsTemplate property to MenuItem API objects

Add boolean property (defaults to `true`) `iconsIsTemplate` to `MenuItem` objects
to allow for proper display of icons in Mac OS X (Yosemite) Dark Menus.

When `iconsIsTemplate` is set to true, the `icon` is treated as a "template" and
the system automatically ensures proper styling according to the various states
of the menu item (e.g. dark menu, light menu, etc.).
Template images should consist only of black and clear colours and can use the
alpha channel in the image to adjust the opacity of black content.

See [Dark Menus in AppKit Release Notes for OS X v10.10](https://developer.apple.com/library/mac/releasenotes/AppKit/RN-AppKit/#10_10DarkMenus)
and [`NSImage setTemplate:`](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSImage_Class/index.html#//apple_ref/occ/instm/NSImage/setTemplate:).

On Linux and Windows setting the property has no effect.

FIX rogerwang/node-webkit#2773
Merge pull request #2775 from mrfabbri/fix-osx-dark-mode

Add API methods and support for styling of icons (Tray, MenuItem) under Mac OS X (Yosemite) Dark Mode