Revert "Fixes an FTBFS in the Qt4 build"

This reverts commit bd9510f926aeb7bbdb11b428ab2c26e82861be37.
Revert "Merge branch 'use-qt5-qiconloader'"

This reverts commit b18046858e4911c58de1dbfd57e8177bdd65ae0b, reversing
changes made to 0d80985769ab05691ab3c8768ca63f7b6c74feb4.
Merge branch 'use-qt5-qiconloader'

* use-qt5-qiconloader:
  Honors /usr/share/pixmaps when searching for icons.
  Adds xpm icon support
  Adds Qt5 QIconLoader for the Qt5 build

(cherry picked from commit b18046858e4911c58de1dbfd57e8177bdd65ae0b)
Fixes an FTBFS in the Qt4 build

Change-Id: Ic0d6448355be6c94fb36bd134fdfb36b25790c88
Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
(cherry picked from commit bd9510f926aeb7bbdb11b428ab2c26e82861be37)
Allows QIconLoader to search in multiple theme directories.

Even when QIcon::themeSearchPaths() returns a list QIconLoader doesn't
honor it. It will search for an icon in the first directory it finds (that
has the current theme).
QIcon still doesn't support a theme that is spread across several
directories. And it's a fundamental feature needed to support user
installed icons, e.g., the hicolor theme.
if QT_VERSION < 5 load the qt4 header
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.