Add meta-fields for media assets

[tidyui]

Add the standard meta-information fields

• Title
• Alt Text
• Description

to media assets. Also add support for globally defined custom properties for media assets. These should be defined like this in Startup.cs.

App.MediaTypes.MetaProperties.Add("External Id");

And can then be updated and consumed through the Api like this

media.Properties["External Id"] = "1234562;
var externalId = media.Properties["External Id"];