object: made is() stricter and added extends()

Currently some plugins had/have issues with predicates that check
if active view is a docview to perform certain operations like draw
in the case of minimap or lineguide. Since is() was checking the
entire inheritance tree it was returning true for views that inherit
from the same parent, which caused CommandView to be matched along
DocView, etc... This change does the following to solve the issue:

* Make Object:is() only match the top level parent of the object which
  is more in line with what one would expect from a method named 'is'.
* Introduces Object:extends() which keeps the same functionality that
  Object:is() offered before.

data/core/object.lua

@@ -21,6 +21,14 @@ end
 
 
 function Object:is(T)
+  if getmetatable(self) == T then
+    return true
+  end
+  return false
+end
+
+
+function Object:extends(T)
   local mt = getmetatable(self)
   while mt do
     if mt == T then