Add `vsync` property to windows for native targets

[Starmapo]

Currently, setting a window's VSync value is only possible from the Project.xml file. This pull request adds the vsync property to lime.ui.Window, which allows for getting and setting the VSync value during runtime. Useful if you want to let the user decide if they want VSync enabled.

This is only implemented for native targets, and doesn't work for Flash or HTML5 applications, though I don't know if they support VSync.

[dimensionscape]

Which platforms have you tested this on?

[Starmapo]

Which platforms have you tested this on?

Windows.

[dimensionscape]

Thank you. This is a nice addition.