Blender Git Loki

Fix saving images from temp Image Editor failing

Steps to reproduce were:
* Ensure //Render//->//Display Mode// is //New Window//
* F12
* In the opened Image Editor, Alt+S to save the image
* Save the image
The saving would fail silently.

Issue was that wm_handler_op_context() would fail to find the correct
area to activate, as the wrong window was active in context. So allow
overriding this window and do so when creating the file-select handler.