Fix handling QT_IM_MODULE=none

Was added with 19a39a4e to allow no input context to be created.
Broken by commit 24c10b0b

. Especially if compose input context didn't
get compiled, the first input context found was loaded.

Also made Xcb integration use compose plugin by default.

Change-Id: I992eaa8b383320e4ab725bb7b79f561f4f841458
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>

src/gui/kernel/qplatforminputcontextfactory.cpp

@@ -83,7 +83,7 @@ QPlatformInputContext *QPlatformInputContextFactory::create()
     QString icString = QString::fromLatin1(qgetenv("QT_IM_MODULE"));
 
     if (icString == QLatin1String("none"))
-        icString = QStringLiteral("compose");
+        return 0;
 
     ic = create(icString);
     if (ic && ic->isValid())

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -384,6 +384,9 @@ QXcbConnection::QXcbConnection(QXcbNativeInterface *nativeInterface, bool canGra
         qunsetenv("DESKTOP_STARTUP_ID");
 
     sync();
+
+    if (qEnvironmentVariableIsEmpty("QT_IM_MODULE"))
+        qputenv("QT_IM_MODULE", QByteArray("compose"));
 }
 
 QXcbConnection::~QXcbConnection()