Add a filter debugger

[madmaxoft]

Add a dialog that would allow one to "debug" template filters:

• show a list of songs, together with info whether song matches or not.
• for each filter tree node, show whether the selected song satisfies the node or not.

[madmaxoft]

Not a standalone dialog as such, but a pane in DlgSongs that opens on specific command, takes DlgSongs' selection and displays the filter tree with color-coded match / non-match on each level of the tree.