Handle Select fieldtype in product filters

[SamuelHelbling]

Currently, it is only possible to use fields of type Link or Multiselect as filters for article groups. With this pr, it is now also possible to use fields of type Select as filters. The values displayed are the defined options of the field. This also works for custom fields. The change in the code is minimal, but the benefit for the user is big. The following issues have been fixed:

• Website Specifications for Item Groups are not displayed #244
• Feature Request: Filters using custom field from website Item Doctype #181

Before:

After:

Setup:

1. Create a new custom Select field for the website item doctype with options or use an existing Select field:
   

2. Define the Select field as an item filter in the website group:
   

[SamuelHelbling]

@rohitwaghchaure backport v-15

[SamuelHelbling]

Hi @rohitwaghchaure , just following up on this PR, it’s ready whenever you have a chance to review or merge. Let me know if any changes are needed!