Properly show labels from broken references in Cross-references dialog

Properly show labels from broken references in Cross-references dialog

Fix for bug #12456.

The labels are transmitted from Buffer to GuiRef by reference of refs_
in the getLabelList function. Previously, only one string was
transmitted. But I needed both the formatted string, e.g. "x enu:test"
or "Missing: enu:test", as well as the plain label, e.g. "enu:test".
The former is for the list of labels to choose from in GuiRef and the
latter for the label as shown in the line edit that contains the plain
label in order to create a new reference from it. Transmitting both is
what the pair achieves.