search: Tweak "No Apps Found" placeholder row
The current placeholder that appears when no providers are available has some problems:
- The row is activatable, but this is unnecessary
- The row has no padding
- The "No Apps Found" string doesn't use header capitalization
Fix this by using a GtkListBox placeholder widget, with margins and with header capitalization in the string.
With this change, the row now looks the same as the placeholder rows from "Search > Search Locations > Custom Locations" and "Privacy > Location/Microphone/Camera".
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Edited by Automeris naranja