Redesign selection filter as a normal QDockWidget

Currently the selection filter widget is designed as a QListWidget, and it has white background while other panels have grey background (in light mode, in dark mode the colours are different too). While other panels have a very comfortable look, the tick boxes in the selection filter are all clustered together.

This isn't a beautiful layout. I can see the convenience of defining a QListWidget, but the selection filter is a dock panel just like palettes, inspector and mixer. They should have the same general look.

QListWidget is used when a dialog needs to include a list, like the one in the Instruments dialog:

But if the whole dialog is a list, a design of this should be more careful. Especially when the dialog is supposed to be placed beside dialogs of the same kind (in this case, dock widgets), a inconsistent look can upset users each time they look at it.