Blender Git Loki

WM: various changes to file writing behavior

Saving with only a filename (from Python) wasn't being prevented,
while it would successfully write the file to the working-directory,
path remapping and setting relative paths wouldn't work afterwards
as `Main.filepath` would have no directory component.

Disallow this since it's a corner case which only ever occurs
when path names without any directories are used from Python,
the overhead of expanding the working-directory for all data saving
operations isn't worthwhile.

The following changes have been made:

- bpy.ops.wm.save_mainfile() without a filepath argument
fails & reports and error when the file hasn't been saved.

Previously it would write to "untitled.blend" and set the
`G.main->filepath` to this as well.

- bpy.ops.wm.save_mainfile(filepath="untitled.blend")
fails & reports and error as the filename has no directory component.

- `BLI_path_is_abs_from_cwd` was added to check if the path would
attempt to expand to the CWD.