Correct doxygen paths and header include guards after last changes

Add widgets/intern, improve naming, move around files

After feedback from Campbell.

File structure now looks like this:
/windowmangager
...
/widgets
WM_widget_api.h
WM_widget_library.h
WM_widget_types.h
wm_widget_wmapi.h
/intern
wm_widget_intern.h
wm_widget.c
wm_widgetgroup.c
wm_widgetmap.c
/widget_library
...

Cleanup includes

Move wmWidgetMapType_Params struct to WM_widgets_types.h

Correct widget map type flag name

Move wmWidget structs and defines into own header files

For external structs/defines we now have windowmanager/widgets/WM_widgets_types.h.
Also some cleanup.

Remove wm_widgets.c!

Used to be a big file, content is now split into multiple files in windowmanager/widgets.

Move stuff from wm_widgets.c file to proper places

Move widget declarations/defines into right header

Move wmWidgetGroup(Type) functions into own file

Move wmWidgetMap(Type) functions into own file

Think one file for wmWidgetMap and wmWidgetMapType is enough.

Move wmWidget functions into own file

Moved widget functions from windowmanager/intern/wm_widgets.c to windowmanager/widgets/wm_widget.c. Added WM_widgets_api.h for external widgets API and wm_widgets.h for window manager API.

Split wm_generic_widgets.c into separate files per widget type

New files are now located under windowmanager/widgets/widget_library/xxx_widget.c. Added two headers, wm_widgets_intern.h and widget_library.h (which only contains external declarations and is only used as #include in WM_api.h).

wm_generic_widgets.c is gone now.

Move widget geometry files to windowmanager/widgets/widget_library

Moved files to new location, renamed them to geom_xxx_widget.c/widget_geometry.h, added license blocks, doxygen info, etc.

Use general exit callback called after widget tweaking

Will be used to free data and reset property to initial value.

Always activate widget when starting to drag (not only if handling is available)

Transform manipulator: New widget for trackball rotation

Adds a new (partially invisible) widget to the rotation transform manipulator that triggers trackball rotation. It is activated by clicking inside the outer white circle, but not on any other axis. For better feedback, a slight white overlay is drawn on mouse hover.

Fix Blenderplayer not compiling

Merge branch 'master' into wiggly-widgets

Conflicts:
source/blender/blenloader/intern/readfile.c
source/blender/blenloader/intern/versioning_270.c
source/blender/editors/include/ED_transform.h
source/blender/editors/interface/resources.c
source/blender/editors/space_sequencer/sequencer_draw.c
source/blender/editors/transform/transform_manipulator.c
source/blender/gpu/intern/gpu_buffers.c

Work in progress changes to py initialization

Various minor fixes to initializing the Python rna subclass.
With remaining issue that we can't immediately call the keymap_init callback (which is expected).