Build system maintenance

* Makes ${QTXDG_LIBRARIES} also hold the dependencies libs
     ${QTXDG_LIBRARY} holds the qtxdg library itself. ${QTXDG_LIBRARIES}
     should hold qtxdg and all it's dependencies.
     See: http://www.cmake.org/Wiki/CMake:How_To_Find_Libraries

* Qt4: Replaces ${QT_LIBRARIES} by ${QT_module_LIBRARY}
* Qt5: Adds ${QTXDG_LIBRARY_DIRS} to the linker search paths
Updates the cmake_minimum_required() version

CMake v2.8.5 is the minimum for Qt4 and Qt5. In the Qt5 build the minimum
version was lowered to v2.8.3 which didn't make any sense.
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