Disable deletion if a terminal can't be deleted.

In case of user try to delete a terminal element who is bridged
or belong to a physical terminal with more than one level, the deletion
is aborted to avoid mistake in the terminal strip parent of the terminal
element. A dialog is opened when the deletion can't be to explain to
user what to do for enable the deletion.

sources/undocommand/deleteqgraphicsitemcommand.h

@@ -34,10 +34,10 @@ class DeleteQGraphicsItemCommand : public QUndoCommand
 	public:
 		DeleteQGraphicsItemCommand(Diagram *diagram, const DiagramContent &content, QUndoCommand * parent = nullptr);
 		~DeleteQGraphicsItemCommand() override;
+		static bool hasNonDeletableTerminal(const DiagramContent &content);
 
 	private:
 		DeleteQGraphicsItemCommand(const DeleteQGraphicsItemCommand &);
-
 		void setPotentialsOfRemovedElements();
 		Terminal *terminalInSamePotential(Terminal *terminal, Conductor *conductor_to_exclude);