Implement a delegate class for indexeddb object stores

* Implement functions of export/import/delete `ItemDrops` and `EquipmentDrops`
Fix the display format of the date of equipment drops
Change the layout of 'Drops' tab

* Remove 'Clear Items' button and 'Clear Equipments' button
* Change some labels
* Modify 'Monster Database' tab

* Add export/import/delete UI for 'Item Drops' and 'Equipment Drops'
* Change element Ids
* Fix typos
Remove the event handlers of 'Clear Items' and 'Clear Equipments' on 'Drops' tab
Modify UI code for 'Monster Database' tab