Introduce TaskbarProgress::stop()

Change-Id: Id37e6c5e41ab196cd72148b3242332796b1c54ef
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Tidy up the QML extension plugin

Remove redundant qwinextrasplugin.h, fix Q_PLUGIN_METADATA(), and
rename qwinextrasplugin.cpp to plugin.cpp to follow the established
convention in qtdeclarative/src/imports.

Change-Id: I6f2f710283cfdd43fa15b01dcc63bcb65ab1eca7
Reviewed-by: Caroline Chao <caroline.chao@digia.com>