Fix & Feat: optional direct fetch, filter & limit to get_items()

[Madcheese101]

Feat: option to change how filtering and fetching items work, now you can switch to direct item data fetch and filter from the database instead of loading all items when logging in to POS and then filtering the loaded items.

ItemSelector.vue:

• add onchange event for item_group & search bar so that if the new feature is enabled it points to search_onchange() and filters from the source.
• when enter key is pressed now it looks for search value in serial_no_data of the first item in search list. If found it will be added to invoice list.
• filter_items method now only works if the new feature is disabled.
• fixed text typo

posapp.py:

• add search_serial_or_batch_or_barcode_number method
• add get_conditions method.
• edit get_items() method to look for serial number, barcode, batch number, and item_code if the new feature is enabled.
• add new search conditions and limit statement to get_items() method if the new feature is enabled.
• add warehouse filter to fetching serial_no_data to fetch only the serial numbers for the current pos profile warehouse.
• order items by item_name instead of item code.
• fix typos.

Invoice.vue

• add new validation to validate() method to check for items with zero quantity in the invoice and shows error if true.
• selecting or deselecting serial numbers now reflects quantity as selected serial number count instead of just showing a warning
• fix typos

other: fix typos