Specify generic QPA plugins via env variables

Introduce a mechanism to load comma separated generic QPA plugins via
exported environment variables. (Drastically simplifies QPA usage in custom
environments where a variety of plugins could be used and heuristics are
consequently a poor fit)

Change-Id: I292f3fbfc8b8ad2b4f416984ef0a10b9b59de531
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/kernel/qguiapplication.cpp

@@ -711,6 +711,10 @@ void QGuiApplicationPrivate::init()
         argc = j;
     }
 
+    // Load environment exported generic plugins
+    foreach (const QByteArray &plugin, qgetenv("QT_QPA_GENERIC_PLUGINS").split(','))
+        pluginList << plugin;
+
     if (platform_integration == 0)
         createPlatformIntegration();