Add UI controls for spherical cursor controls and adjustment

[andy-sweet]

The third selection mode is effectively a way to make manual adjustments to the transform of the spherical sphere.

I find these adjustments hard to make in the canvas only. It is also hard to discover how to switch transforms modes (i.e. translation, scale, rotation) after the instructions have disappeared (i.e. after selecting something).

Having UI controls would solve both issues by making these controls more discoverable and easier to fine tune without needing to manipulate small handles in the scene.

[TeunHuijben]

Progress on this issue:

• I made the selection instructions always visible, so one can always see them, also after making a first selection
• I added UI buttons for interactions with the spherical cursor. One can now change the selector size and select cells with a UI button, instead of using keyboard shortcuts. However, for now, I only made them available on iPad (where they are necessary), to not clutter the leftSideBar with too much UI
• having a preview of the cells selected by the selector in yellow, before actually selecting them is already a great improvement in interacting with the selector