qmlpreview: prepare plugin to have an external client

- export the plugin andd add client creator API to be able to implement
  external debugtranslationclients
- use the private/qqmldebugtranslationprotocol_p.h from Qt5::QmlDebugPrivate
- have a basic client implementation which cares about changing language
- remove out of date elide warning implementation
- remove old ui
- remove the menu entry

The old client implementation is incompatible with the will be introduced
service and there was never an officially released compatible service.
Therefore we can remove the old client implementation without causing
any harm.

Change-Id: I26b5e8a99ba30ae6377443b3fffb05901b1cac28
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmlpreview/tests/qmlpreviewplugin_test.cpp

@@ -31,11 +31,10 @@
 #include <QtTest>
 #include <QVariant>
 
-Q_DECLARE_METATYPE(QmlPreview::Internal::TestFileLoader)
-Q_DECLARE_METATYPE(QmlPreview::Internal::TestFpsHandler)
+Q_DECLARE_METATYPE(QmlPreview::TestFileLoader)
+Q_DECLARE_METATYPE(QmlPreview::TestFpsHandler)
 
 namespace QmlPreview {
-namespace Internal {
 
 QmlPreviewPluginTest::QmlPreviewPluginTest(QObject *parent) : QObject(parent)
 {
@@ -90,5 +89,4 @@ void QmlPreviewPluginTest::testFpsHandlerProperty()
     handler(stats);
 }
 
-} // namespace Internal
 } // namespace QmlPreview