offscreen plugin: eradicate Q_FOREACH loops

... by replacing them with C++11 for-each loops.

In clearHash(), replace iteration over QHash::keys() with
iteration over the hash itself. Also use the new const-
iterator overload of QHash::erase() to save one attempted
detach (in QHash::find()).

Mark the plugin as QT_NO_FOREACH.

Saves almost 2KiB (1.4%) in text size on optimized GCC 6.1 Linux
AMD64 builds.

Change-Id: I9bb3154130687c0061ea09b04ab5f627a4d2296c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>