[Qt] Use Qt5.1 supportedMimeTypes methods.
https://bugs.webkit.org/show_bug.cgi?id=111185

Reviewed by Jocelyn Turcotte.

Uses the supportedMimeTypes() methods from QImageReader and QImageWriter when available.

* platform/MIMETypeRegistry.cpp:
(WebCore::initializeSupportedImageMIMETypes):
(WebCore::initializeSupportedImageMIMETypesForEncoding):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145066 268f45cc-cd09-0410-ab3c-d52691b4dbfc