feat: Allow tool palettes to stack horizontally in addition to the default vertical stacking

[seankmartin]

Summary

This PR introduces a horizontal stacking option to tool palettes, which can be toggled by clicking the button in the palette menu topbar on a per-palette basis. Vertical stacking remains the default. This allows a bit more customisation, and can be particularly helpful if the tool palette is below the viewer like in the below:

User interaction

In neuroglancer, this setting is available on all tool palettes. Palettes stack vertically by default, and can be manually changed to stack horizontally in the tool palette menu. This is also stored in the state.

Implementation

The primary change is to the tool palette menu, to introduce a setting which changes the widget stacking direction. In addition, there is a slight change to where the tool palette drop zone is positioned. This is so that the horizontal stacking can take up the full space of the element along the x-direction. In horizontal stacking, the drop-zone remains at the bottom of the palette menu, the same as for vertical stacking.

[aranega]

All looks good and works well on my side :) 🙏 @seankmartin !