add base document init method to create new configurable for each document

this should fix configurables pollution when opening multiple documents
at the same time, e.g. when clipping page images in Evernote plugin.

frontend/document/document.lua

@@ -50,18 +50,22 @@ local Document = {
 
     -- flag to show that the document needs to be unlocked by a password
     is_locked = false,
-
-    configurable = Configurable,
 }
 
 function Document:new(o)
     local o = o or {}
     setmetatable(o, self)
     self.__index = self
+    if o._init then o:_init() end
     if o.init then o:init() end
     return o
 end
 
+-- base document initialization should be called on each document init
+function Document:_init()
+    self.configurable = Configurable:new()
+end
+
 -- override this method to open a document
 function Document:init()
 end