Switching from warning types new blank fields should be filled with default values

[dotboris]

Steps to reproduce:

1. Create a border warning
2. Edit that same warning
3. Switch it to a top banner (or bottom banner)

From there, the fields for message, fg and bg colors are empty:

They should default to the usual default values.

Something similar happens when you go from banner (top / bottom) to border:

Again, things should default to the usual default values.