view: Implement add to playlist in search results

This is for albums and songs for now.
view: Use casefold instead of lower when comparing artists
view: Group artist albums by artists in Search view
view: Show the albums when an artist is selected in Search view
view: Update add to playlist in search results

Add songs of selected artists too.
searchbar: Clear timeout variable in source callback

If the callback is already called, we no longer need to remove it since
it is removed automatically after return.

Removes warnings.
searchbar: Separate dropdown headers from default values

To be consistent with other GNOME Core Apps.
searchbar: Replace "Search By" and "All fields" to "Match" and "All"

To be consistent with other GNOME Core Apps.