Fix debug window on GTK4, plus other issues.

There's no need to go through the request API to make the save button work, so do it with a native file chooser.
There's an error CSS class, so use that instead of our custom one. "working" is the default state, so drop the good-filter CSS class.
Setting the search entry widget text from the preference seems to cause an infinite loop, and I don't know why the pref would change without the widget doing so, so drop that.
Move delete stuff into finalize vfunc.
Moved filter options menu to the 'filter' button, as GtkSearchEntry does not expose the icon as in GTK3.

Opened debug window, set various debug levels, clicked save button and saved log somewhere, entered some text in filter and toggled enable button, right-clicked enable and toggled the Invert and Highlight options.