Fix deprecation warnings about constructing QString from const char*.

Fix deprecation warnings about constructing QString from const char*.

types/qqmlmodelindexvaluetype.cpp: In static member function 'static QString QQmlModelIndexValueType::propertiesString(const QModelIndex&)':
types/qqmlmodelindexvaluetype.cpp:47:56: warning: 'QString::QString(const char*)' is deprecated (declared at qstring.h:649) [-Wdeprecated-declarations]
items/qquickimagebase.cpp: In member function 'virtual void QQuickImageBase::load()':
items/qquickimagebase.cpp:213:49: warning: 'QString::QString(const char*)' is deprecated (declared at qstring.h:649) [-Wdeprecated-declarations]
items/qquickimagebase.cpp:213:88: warning: 'QString::QString(const char*)' is deprecated (declared at qstring.h:649) [-Wdeprecated-declarations]

Change-Id: I03266aba589ade8228bc286cd3f3f237cd06a780
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>