[Shotwell] RAW import speed improvements

[Jens Georg <mail jensge org>]

Hi,

I committed a couple of fixes to master that should speed up raw import 
a bit.

Since I only have RAW images with embedded development available, I 
would appreciate if people with sidecar JPEG (RAW+JPEG) files and RAW 
files without any prior development could test master a bit for any 
regression/improvements.

Note: The heavy RAW lifting still happens in the GUI thread, which 
causes the UI to be awfully sluggish, that's next on the things to 
tackle.

And then I will slow down the import again to make it a bit less CPU 
hogging.

KR