Brand colors are not validated

[danirabbit]

What Happened?

Apps that use invalid values for their brand color can end up with illegible app info pages

Steps to Reproduce

See capnet assist for example

Expected Behavior

We should validate brand colors. According to the spec, this has to be a hex color: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-branding

OS Version

7.x (Horus)

Software Version

Latest release (I have run all updates)

Log Output

No response

Hardware Info

No response