Rendering settings/preview issues

Another set of rendering settings/preview fixes, see https://trello.com/c/8wA2ry0T/65-ui-fixes-rendering-controls-preview
* The preview now makes use of additional space (use the divider below preview to adjust the space available for the preview)
* Show confirmation dialog before resetting rendering settings (either to "imported" or to "owner's")
* "Full range" is always visible, but greyed out for non-integer images
* Pixel type is now "Float" or "Double" instead of "Float precision" or "Double precision"
* Full image viewer: Advanced rendering settings are always shown