Replaces XdgMimeInfo with XdgMimeType

Qt5 has mimetypes support: the QMimeDatabase and QMimeType classes.
QMimeDatabase is cached.
Qt4 has no real mimetype support. I found QtMimeTypes,the father of Qt5
QMimeType/QMimeDatabase. It works (at least) with Qt 4.7.2. It replaces
ligmagic for the libqtxdg Qt4 version. The API is the same as the Qt5 one.
QtMimeTypes is here:
https://qt.gitorious.org/qtplayground/mimetypes

The default for the Qt4 version uses QtMimeTypes. Users that still
want to use the XdgMime/libmagic (e.g. Razor) can use the CMake
configuration option: USE_QTMIMETYES=Off

Note: The Qt4 and Qt5 versions now have different dependencies:
* Qt4 - QtMimeTypes (https://qt.gitorious.org/qtplayground/mimetypes)
* Qt5 - None