Marker it1.

[sohocoke]

PoC basics of a marker overlay. Check if:

• it is a good balance in indicating and being seamless / unintrusive;
• it can serve as the anchor point for a popover-style presentation.

[sohocoke]

blocks: #57

[sohocoke]

Marker remaining bits:

• derive size based on button group frame.
• add placeholder colour selection controls to workspace.

Maybe:

• disallow activation.