A bunch of work on loading plugins.

CMakeLists.txt

src/calamares/CMakeLists.txt

src/calamares/Module.cpp

src/calamares/Module.h

src/calamares/ModuleLoader.cpp

src/calamares/ModuleLoader.h

src/calamares/Settings.cpp

src/calamares/YamlUtils.cpp

src/calamares/YamlUtils.h

src/calamares/viewpages/AbstractPage.h

src/calamares/viewpages/PagePlugin.h

src/libcalamares/CMakeLists.txt

src/modules/greeting/CMakeLists.txt

src/modules/greeting/GreetingPage.cpp

src/modules/greeting/GreetingPagePlugin.cpp