Enable building all of the available platform plugins

There should be options to turns plugins on and off. Following the conventions of other plugin types that means -platform-xxx and -no-platform-xxx switches. The list of available plugins can be determined by looking in src/plugins/platforms/*

Wayland may be a special case because we need a configure option to enable it or not but the actual plugin lives somewhere else. So for wayland keeping -wayland and -no-wayland might make sense (unless all of the wayland-specific code is somewhere else, in which case configure shouldn't mention wayland at all).

It may be worth keeping the -xcb and -no-xcb switches for compatibility but they will be synonymous with -platform-xcb and -no-platform-xcb.

By default, any plugin that can build should build (using configure/platform tests). The logic in src/plugins/platforms/platforms.pro should use the list configure assembles to decide which plugins to build.

As well as selecting the plugins to build, configure needs a way to select the default platform that is used. Some logic should select a suitable default from the enabled platforms (using a weighted list) and the user can override this if needed. src/gui/kernel/qguiapplication.cpp should not hard-code defaults but should instead use the default selected by configure. It might be useful to compile this default into qconfig.cpp so that it can be overridden using qt.conf or by performing a binary patch on libQtCore.so.

If the user has not requested any platform plugins and only the minimal plugin will be built, configure should stop. The user must pass -platform-minimal to allow a build that does not have any other platform plugins enabled (may have consequences for wayland builds depending on how that is worked out).