Casefold strings instead of lowercaseing when searching

Tracker's fn:lower-case calls u16_tolower or you_strToLower depending
on whether its libunistring or libicu is being used, which may case
Unicode strings to be "lowercased" differently than JavaScript's
String.prototype.toLowerCase.

In contrast tracker:case-fold calls g_utf8_casefold. Lets compare
outputs from the same function for the sake of predictability.

Moving the casefolding slightly lower down in the application has the
pleasant side-effect of not messing up the search string in the UI.
eg., if you enter "PASSAGE", preview a document, and come back to the
overview, then the search string in the UI would not become "passage".

https://bugzilla.gnome.org/show_bug.cgi?id=722246
documents: Add debug messages for failing to use a thumbnail file

https://bugzilla.gnome.org/show_bug.cgi?id=720533