Bugfix: Paste pending rendering settings

Bugfix: "Paste and Save" actions (TreeViewer and DataBrowser) didn't work with pending (not yet saved) rendering settings; see: https://trello.com/c/8wA2ry0T/65-ui-fixes-rendering-controls-preview

This is actually just a small bug but needed lots of code modifications, sorry for that.
@jburel : Would be great if you could check if this is a legitimate way of doing this, especially see ```RenderingSettingsSaver```; how it works in short: Load the user's rendering settings for the reference image (in order to be able to restore them again afterwards); apply the 'pending' rendering settings to the reference image; paste the saved reference image's rendering settings to the target images, restore the original user's renderings settings for the reference image again.

Testing: Modify the rendering settings; don't (!) save them; copy the rendering settings and apply ("Paste and Save") them to other images via the context menu of the TreeViewer and the central DataBrowser. 

--no-rebase