Add Statistic plugin (#1581 Amount of hours spent on a book)

frontend/document/credocument.lua

@@ -156,6 +156,10 @@ function CreDocument:getCoverPageImage()
     end
 end
 
+function Document:getProps()
+        return self._document:getDocumentProps()
+end
+
 function CreDocument:getWordFromPosition(pos)
     local word_box = self._document:getWordFromPosition(pos.x, pos.y)
     DEBUG("CreDocument: get word box", word_box)

frontend/document/pdfdocument.lua

@@ -8,6 +8,7 @@ local DEBUG = require("dbg")
 
 local PdfDocument = Document:new{
     _document = false,
+    is_pdf = true,
     dc_null = DrawContext.new(),
     options = KoptOptions,
     koptinterface = nil,

frontend/document/picdocument.lua

@@ -4,6 +4,7 @@ local pic = nil
 
 local PicDocument = Document:new{
     _document = false,
+    is_pic = true,
     dc_null = DrawContext.new()
 }