add undelete for entities

BACKSPACE in combination with editmeta (LCTRL) now undeletes the last
deleted entity. BACKSPACE in combination with editmeta2 (LSHIFT) brings
up the menu "deleted entities".

The edit menu now has a submenu "deleted entities" which lists all
deleted entities and undeletes the selected one.

undelent [<index>]
  If no index is specified, the last deleted entity is restored. If an
  index is given, the specified entity from the list of deleted entities
  is restored.

getdeletedentities
  Returns a list of deleted entities. Each line contains the entity type,
  the position and all seven attributes.

unlistdeletedentity [<which>]
  If <which> is a number, the specified entry from the list of deleted
  entities gets removed. If <which> is "all", the whole list of deleted
  entities gets deleted. If <which> is "last" the last deleted entity is
  removed from the list of deleted entities.
  Returns the number of entities on the list of deleted entities (even if
  the parameter <which> was unknown or not specified).

DOCREF undelent getdeletedentities unlistdeletedentity

source/src/rendermodel.cpp

@@ -166,6 +166,7 @@ void deletemapmodelslot(int *n, char *opt) // delete mapmodel slot - only if unu
         entity &e = ents[i];
         if(e.attr2 == *n)
         { // delete entity
+            deleted_ents.add(e);
             memset(&e, 0, sizeof(persistent_entity));
             e.type = NOTUSED;
             deld++;