Factor out QPlatformTheme from QPlatformPlugin.

Factor out QPlatformTheme from QPlatformPlugin.

Implement QCocoaPlatformTheme. The menus and dialog
API is moved from the platform to the theme plugin.
(Both APIs contain references to QtWidget classes,
which we are working towards removing.)

The theme plugin is created after the platform
plugin, first by asking the platform plugin, then
by looking for a separate plugin if the platform
does not specify a theme.

Initial-patch-by: Morten Sorvig <msorvig@trolltech.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Change-Id: I2778cdd3a205c4ce35ead93e39fe6b4cd58a39f9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>