Search plugin should allow for a custom placeholder

[markfee]

The manifest for the search plugin implies that a label can be provided.

I hoped that setting this would alter the placeholder text from the default "Search", but it seems to have no effect. Checking the code, I can see that "Search" is hard coded as the placeholder.

Definition of Done:

Clarify that the intention that setting a label in the manifest should alter the placeholder text.
Fix the issue so that the label appears as the placeholder when set, OR add a new entry to the manifest - I suggest placeholder - that affects the placeholder text.