Popup naming conventions
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Right click context popup on drawing area:
	- /popups/popup-obj-TYPE
		floating object (nor part of any atomic group); TYPE is substituted
		with object type, e.g. polygon

	- /popups/popup-obj-misc
		called if popup-obj-TYPE failed

	- /popups/popup-user-grp-PURPOSE
		on a group with no role; PURPOSE is substituted with the purpose
		attribute of the group

	- /popups/popup-user-grp-unknown
		Same, but if the purpose attribute is empty or does not exist, it is
		substituted with "unknown".

	- /popups/popup-ROLE
		Group with role=ROLE, ROLE must be one of the cschem-defined group roles,
		e.g. symbol, terminal, etc.

	- /popups/sheet
		Clicked on an empty part of the drawing area of a sheet

	- /popups/symbol-as-sheet
		Clicked on an empty part of the drawing area of a sheet when in symbol
		editing mode

	- popups/none
		None of the above (e.g. clicked outside of drawing area)