Add support to QMAKE for specifying default plugin for modules

Some modules, like QtSensors, have a plugin based architecture. And for many platforms, e.g iOS and Android, there is only one plugin to choose from. Nevertheless, when the user writes his pro file, he would have to add this plugin to his QTPLUGINS variable, which will have different value/plugin for each platform. This is error prone, and typically leads to "debug pro file everywhere" scenarios.

It would be really nice if QMAKE could do this automatically, that is, fill in QTPLUGINS with the default plugins for each module requested. The modules could e.g set QMAKE_DEFAULT_QTPLUGINS or similar.

Extra problematic is the fact that Android does this already today through their ant files (working around qmake). This means that an app that builds and runs on Android (by only doing QT += sensors) will not work on iOS.