Adds an XdgMimeType class

For Qt5, the internal QtMimetypes, will be used.
For Qt4, QtMimeTypes, will be used. QtMimeTypes is the project that gave
birth to the Qt5 QtMimeTypes. The main repository is here:
	https://qt.gitorious.org/qtplayground/mimetypes
Deprecates XdgMimeInfo and XdgMimeInfoCache

Just use QMimeType or XdgMimeType.
QMimeDatabase is cached. There's no need to use XdgMimeInfoCache.