Move "items under cursor" out of ContextMenu

The context menu was calculating which items were under the cursor based on the last time the mouse was on the canvas. This makes sense for the right click on the canvas. But no sense at all for the layers and objects dialog.

We move the calculation for "items under cursor" out of the ContextMenu and let the item-tool tell us about the items under the cursor.

Then we let the layers dialog talk about the items under its cursor.

Fixes #5047