Determine future of the workspace cursor

[gonfunko]

#491 disables the workspace cursor (the indicator of a specific location on the workspace, not the cursor that's used to move around between blocks in general) because it doesn't work with the new INavigable approach. We could replace it with most likely a class that maintains a position on the workspace and draws an indicator on an appropriate layer, or remove it entirely in favor of insert-in-move-mode, since choosing a location for a newly inserted block is the only purpose of the workspace cursor at present AFAIK.

[microbit-matt-hillsdon]

Another important usecase is triggering the workspace context menu.

[rachel-fenichel]

Remaining work for v12:

• Check whether it is possible to trigger the workspace context menu in the current state.
• If not, add a single navigation stop to make it possible. Do not allow moving the workspace cursor in x/y
• If yes, move this to Feature Complete

[maribethb]

@RoboErikG can you check if it's possible to trigger the context menu with ctrl+enter on the latest changes, please

[RoboErikG]

It is possible to trigger the menu by pressing 'w' to focus on the workspace and then 'cmd + enter'.

Side note: 'shift + tab' doesn't work after the marker was removed, but can still use 'w'.