weaponplanner: add upgrade links

- add 'children' field to weapon JSON, with list of dicts
  containing 'id' and 'name' keys for weapons that can be
  upgraded from current weapon.

mhapi/db.py

@@ -301,7 +301,8 @@ class MHDB(object):
 
     def get_weapon(self, weapon_id, get_components=False):
         weapon = self._query_one("weapon", """
-            SELECT * FROM weapons
+            SELECT items._id, items.name, items.buy, weapons.*
+            FROM weapons
             LEFT JOIN items ON weapons._id = items._id
             WHERE weapons._id=?
         """, (weapon_id,), model_cls=model.Weapon)
@@ -320,6 +321,18 @@ class MHDB(object):
             self._add_components(weapon)
         return weapon
 
+    def get_weapons_by_parent(self, parent_id, get_components=False):
+        weapons = self._query_all("weapon_by_parent", """
+            SELECT items._id, items.name, items.buy, weapons.*
+            FROM weapons
+            LEFT JOIN items ON weapons._id = items._id
+            WHERE weapons.parent_id=?
+        """, (parent_id,), model_cls=model.Weapon)
+        if get_components:
+            for weapon in weapons:
+                self._add_components(weapon)
+        return weapons
+
     def get_armors(self):
         return self._query_all("armors", """
             SELECT items._id, items.name, items.buy, armor.*