FIX: Numerous fixes

This also fix old issues:

* Top border is only the top border of the selected area
  (and not he top border of each cell in the area)
* Same for bottom, left and right borders

Factored out all the border related issues to a new file

base/src/user_model/mod.rs

@@ -1,5 +1,6 @@
 #![deny(missing_docs)]
 
+mod border;
 mod border_utils;
 mod common;
 mod history;