Actually normalize page titles correctly in MWLinkTargetInputWidget

matchingPages contains titles with spaces, but .toString() returns titles
with underscores, so normalizing with .toString() is incorrect. Use
.getPrefixedText() instead.

This caused a bug in the link dialog where, if the link was an internal
link pointing to an existing page with a space in its name, the "New page"
section would be displayed but would be empty, and the link's target would
be displayed in the "Existing page" section but colored red.

Change-Id: I59d8b87b996d80da1b25124b942bd2b72ef50845
Merge "Actually normalize page titles correctly in MWLinkTargetInputWidget"