[libcalamaresui] Use PYTHONQT_INCLUDE_DIRS

- document new variable from the CMake module
 - use it in libcalamaresui to simplify #include'ing the
   header for the "all" extension.

Suggested by Denis Proskurin.

CMakeModules/FindPythonQt.cmake

@@ -2,6 +2,9 @@
 #
 # Sets PYTHONQT_FOUND, PYTHONQT_INCLUDE_DIR, PYTHONQT_LIBRARY, PYTHONQT_LIBRARIES
 #
+# Also sets PYTHONQT_INCLUDE_DIRS to add whatever directories
+#   that are needed for extensions.
+#
 
 # Python is required
 find_package(PythonLibs)

src/libcalamaresui/CMakeLists.txt

@@ -48,7 +48,8 @@ endif()
 
 if( WITH_PYTHONQT )
     include_directories(${PYTHON_INCLUDE_DIRS})
-    include_directories(${PYTHONQT_INCLUDE_DIR})
+    # *_DIRS because we also use extensions
+    include_directories(${PYTHONQT_INCLUDE_DIRS})
 
     list( APPEND calamaresui_SOURCES
         modulesystem/PythonQtViewModule.cpp

src/libcalamaresui/modulesystem/PythonQtViewModule.cpp

@@ -31,7 +31,7 @@
 #include "JobQueue.h"
 
 #include <PythonQt.h>
-#include <extensions/PythonQt_QtAll/PythonQt_QtAll.h>
+#include <PythonQt_QtAll.h>
 
 #include <QDir>
 #include <QPointer>