djvu: Fix case sensitive search

Closes https://github.com/mate-desktop/atril/issues/41
Closes https://github.com/mate-desktop/atril/pull/74

This commit is based on Aleksi Blinnikka one from #74
Changes to make possible compiling in GTK3 mode. Needs --disable-caja flag set!
Merge pull request #76 from Glorf/master

GTK3-compatibility fixes.