Raw conversion: Create preview instances for raw images

[harri]

For now it is OK that these are created always in default volume.

• The copy is also used for exporting if the quality is high enough (e.g. bit depth and color space of exported image is the same). If this is not the case, the raw image is directly converted to exported image
• If the copy is of 1/2 resolution, the zoom settings are anyway show like they would be shown for full resolution image. If user selects to use 100% zoom the raw image is read and used instead.

If color settings change the copy images will be deleted and new ones created.

[harri]

Postponed to next release.

[harri]

This has been implemented in 0.6.0 branch