[Bf-codereview] Drag to close areas (issue 6269044)

[jorge at lunarworkshop.com]

Reviewers: bf-codereview_blender.org,

Description:
Drag to close areas

Please review this at http://codereview.appspot.com/6269044/

Affected files:
    M    .
   M     source/blender/editors/screen/area.c
   M     source/blender/editors/screen/screen_edit.c
   M     source/blender/editors/screen/screen_ops.c
   M     source/blender/makesdna/DNA_screen_types.h