[Feature] Add some keyword only arguments to add_button

[dstansby]

The add_button function has calls that look like this:

add_button(
            "Track tracing",
            self.toggle_methods_layout,
            self.track_seg.toggle_track_panel,
            0,
            1,
            minimum_width=COLUMN_WIDTH,
            alignment=SEGM_METHODS_PANEL_ALIGN,
        )

Out of context of the original signature, the 0 and 1 arguments are a bit myserious. It would be good to convert these to keyword-only arguments in the signature of add_button.