[Qt] Fix compilation of example platform plugin with Qt 5

[Qt] Fix compilation of example platform plugin with Qt 5

Reviewed by Kenneth Christiansen.

Use QLatin1String where appropriate and use the Qt 5 plugin
system with Qt 5.

* examples/platformplugin/WebPlugin.cpp:
(SingleSelectionPopup::SingleSelectionPopup):
(MultipleItemListDelegate::MultipleItemListDelegate):
(MultipleSelectionPopup::MultipleSelectionPopup):
* examples/platformplugin/WebPlugin.h:
(WebPlugin):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121324 268f45cc-cd09-0410-ab3c-d52691b4dbfc