Handle surrogate pairs in TextPage::findText

This should fix issue #853 (closed), but I haven't figured out how to test it yet. It would be great if someone else could make sure that this patch resolves the issue.

In case anyone is wondering, characters in the supplemental plane never have uppercase forms in the basic plane, so if the input character was represented as a surrogate pair then the output character must also be represented as a surrogate pair.