Moved code to replace svg with png to OC.Util

- Moved code that replaces the "svg" extension for the given file to
core as OC.Util.replaceSVGIcon.
- Added unit test for OC.Util.replaceSVGIcon
- Moved "replaceSVG" to OC.Util.replaceSVG and deprecated the global
"replaceSVG" function.
- Added alias for SVGSupport() as OC.Util.hasSVGSupport() (for now)