Add PlaceholderMessage to GridView KCMs

We can add this here in the KDeclarative library component so we don't have to implement it a million times in each individual KCM.

Largely useful in the case of KCMs with a search field, since you can make the view empty by searching for a string that doesn't match anything.

In Colors KCM	In Plasma Style KCM

And here's a video:

2022-04-07_14-14-02.mkv

@teams/vdg