new-style-modules: adding the possebility to import from "freecad"

(this mostly aims at new modules and extension modules which want to use pip)
- any module having problems with nameclashes can use this syntax (from freecad import module)
- current imports still work (backward cobatibility)
- python extension moduels can be installed (pip) to python std path (eg.: site-packages)

- adding app, gui to the new freecad package:
```
from freecad import app
from freecad import gui
```

- syntax for importing c++ extension will not change
```
import FreeCAD as App
import FreeCADGui as Gui
```

src/Ext/CMakeLists.txt

@@ -0,0 +1,6 @@
+# this directory is the entry point for extension modules. Any package
+# installed to a directory "freecad" which is in sys.path can be imported
+# with "from freecad import package" and is checked for a init_gui.py 
+# module, which is import at startup. (FreeCADGuiInit.py)
+
+add_subdirectory(freecad)

src/Ext/freecad/CMakeLists.txt

@@ -0,0 +1,20 @@
+EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c 
+"from distutils.sysconfig import get_python_lib; print(get_python_lib())"
+OUTPUT_VARIABLE python_libs OUTPUT_STRIP_TRAILING_WHITESPACE )
+SET(PYTHON_MAIN_DIR ${python_libs})
+
+SET(EXT_SRCS
+    __init__.py
+)
+
+ADD_CUSTOM_TARGET(freecad_COPY_SOURCE ALL
+    SOURCES ${EXT_SRCS})
+
+fc_copy_sources(freecad_COPY_SOURCE "${CMAKE_BINARY_DIR}/Ext/freecad" ${EXT_SRCS})
+
+INSTALL(
+    FILES
+        __init__.py
+    DESTINATION
+        Ext/freecad
+)

src/Ext/freecad/__init__.py

@@ -0,0 +1,4 @@
+import pkgutil
+__path__ = pkgutil.extend_path(__path__, __name__)
+
+import FreeCAD as app