Visibility of palette based on children elements

Fix for a document's page duplication. Teacher bar's info wasn't quite correct. This is also related to ticket #409 part 2.

Fix for losing content mentioned in the ticket 228. The problem was actually in page deletion algorithm which has been flooding cache with invalid data, and that data was a object to return when loadDocumentScene was called.

Added new system util method: deleteFile.

Update Spanish translation

Incorrect .iss file used for release. Old one deleted.