[Development] Qt platform and theme plugins

[morten.sorvig at nokia.com]

Hello fellow Trolls,

Those of you that follow the commits on Gerrit closely may have noticed the new QPlatformTheme API and wondered what it is. In short, the QPlatformTheme API is there to support deeper platform integration, indepentently of QPlatformIntegration. 

Examples of (current) QPlatformIntegration plugins: xcb, wayland, cocoa, windows
Examples of (possible) QPlatformTheme plugins: kde, gtk, genericunix, cocoa, windows
Notice that for some platforms the integration and theme plugins are tightly coupled. This is up to the platform.

The QPlatformTheme API will be drawn from several sources in Qt 4:
* Various Q_WS ifdefs (system menus, native file/color/font dialogs)
* QGuiPlatformPlugin
* QPlatformMenuBar

QPlatformTheme currently has API for dialogs and system menus. Which APIs belong in QPlatformTheme is up for debate; we want to share as many platform abstractions as possible between QtWidgets and QtQuick, but at the same time some of the APIs might be to tied to the QWidget world.

Morten