Allow "specialized" fields / attributes / parameters / properties / key-value pairs

[jlsjonas]

Related to #45, allow certain extra fields. by default or in settings (allowing adding/removing field-names, with select types (text, date, price and optionally field would be a great start) that would be available for every item, and would preferably have an optional default value.

long-term allow filtering fields to categories and adding extra types such as image/video

default items I'd add:

• Purchase date : date (alternatively: valued)
• warranty period : "month" selector (x months), with default value in settings (for warnings long-term)
• Cost (while you're at it) : price
• Barcode/QR field : text (see also #43)
• Receipt : image (see also #45)
• Temp. location: Storage (see also #47)
• Quantities (see also #38)
• Brand
• Color