fdo#80373 Updated label to read "Image background transparency"

Currently the "transparency" tab only covers image background transparency.
Updated label so that users know that this tab has no impact on the
transparency of the image itself.

Conflicts:
	cui/uiconfig/ui/transparencytabpage.ui

Change-Id: Ib488974b0965ee54288092b1e9ca2dc4c040547c