Add PLUGIN_CLASS_NAME to qtimageformats plugins

Needed for automating static plugin loading.

Task-number: QTBUG-28131

    # ==[ Subject: One line only short meaningful description for logs ]===|

Change-Id: I15365a9ccf1a5b136ea7e01c4530108631498a2a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/plugins/imageformats/mng/mng.pro

 TARGET  = qmng
 
 PLUGIN_TYPE = imageformats
+PLUGIN_CLASS_NAME = QMngPlugin
 load(qt_plugin)
 
 HEADERS += qmnghandler_p.h

src/plugins/imageformats/tga/tga.pro

 TARGET = qtga
 
 PLUGIN_TYPE = imageformats
+PLUGIN_CLASS_NAME = QTgaPlugin
 load(qt_plugin)
 
 HEADERS += qtgahandler.h \

src/plugins/imageformats/tiff/tiff.pro

 TARGET  = qtiff
 
 PLUGIN_TYPE = imageformats
+PLUGIN_CLASS_NAME = QTiffPlugin
 load(qt_plugin)
 
 HEADERS += qtiffhandler_p.h

src/plugins/imageformats/wbmp/wbmp.pro

 TARGET = qwbmp
 
 PLUGIN_TYPE = imageformats
+PLUGIN_CLASS_NAME = QWbmpPlugin
 load(qt_plugin)
 
 HEADERS += qwbmphandler_p.h