npc and places grouped in a entities dict in map to factorise the common processes and similar methods

entity types forgotten
comment updated
the cell is already selected
method renamed
npc list updated when the map is opened
select the cell when the entity is edited
the items are now named entities, because an item is an object in the game
those attributes are instance attributes, instanciated in the __init__
when an entity is deleted, the map must be flagged as unsaved
Merge branch 'entities-refacto' - closes #21