Add Window.setVisibleOnAllWorkspaces API

For platforms that support multiple workspaces (currently Mac and Linux), this allows node-webkit windows to be visible on all workspaces simultaneously.

- Add `SetVisibleOnAllWorkspaces` method to `Window` API objects.
- Add implementation for Mac OS X.
- Add implementation for Aura (Linux).
- Add configuration support via boolean `'window'` subfield `'visible-on-all-workspaces'` in manifest file (see https://github.com/rogerwang/node-webkit/wiki/Manifest-format#window-subfields ).
- Add `Window.canSetVisibleOnAllWorkspaces()` API method returning a a boolean
indicating if the platform (currently OS X and Linux) support `Window` API object
method  `setVisibleOnAllWorkspace()`.
- Add manual test for `SetVisibleOnAllWorkspaces` method to the `Window` API object.

FIX rogerwang/node-webkit#2523