Added MultipleWindows platform capability.

Added MultipleWindows platform capability.

Several platform plugins, like eglfs, kms, etc don't support multiple
windows as there's no system compositor, they're rendering directly to
a single back buffer. By adding a platform capability we'll be able to
provide better error reporting when an application tries to create
multiple QWindows on a single-window platform. Also, QML apps can use
this capability to figure out whether they should create a QWindow for
dialogs / popups / menus, or whether to just create items in the same
scene, that are shown on top of the rest of the content.

Change-Id: I15b8d21ee2bc4568e9d705dbf32f872c2c25742b
Reviewed-by: Andy Nichols <andy.nichols@digia.com>