Support more than 9 sections in BorderImage

BorderImage is limited in what it can accomplish compared to other solutions such as Android's NinePatch. For example, BorderImage cannot currently solve the issue discussed at http://stackoverflow.com/questions/15801110/is-it-possible-to-make-a-9-patch-drawable-for-a-callout-graphic. It should be expanded to support this, or QtQuick should provide a more powerful alternative (perhaps making http://code.qt.io/cgit/qt/qtquickcontrols.git/tree/src/controls/Styles/Android/qquickandroid9patch_p.h public?).