[libcalamaresui] Tidy the CMake bits

- Explain (briefly) what the library is for
 - Explicitly add includes from the non-UI library

src/libcalamaresui/CMakeLists.txt

@@ -1,3 +1,9 @@
+# libcalamaresui is the GUI part of Calamares, which includes handling
+# view modules, view steps, widgets, and branding.
+
+# The UI libs use the non-UI library
+include_directories( ${CMAKE_SOURCE_DIR}/src/libcalamares ${CMAKE_BINARY_DIR}/src/libcalamares )
+
 set( calamaresui_SOURCES
     modulesystem/CppJobModule.cpp
     modulesystem/Module.cpp