Update UI when deleting last point of shape with Path tool (#979)

* Added new DocumentResponse variant

* Update Operation::DeleteSelectedManipulatorPoints to update Layer Tree by delegating deletion to Operation::DeleteLayer. Also emits Operation::DeletedSelectedManipulatorPoints to let editor clear Properties panel

* Update process_message() to deal with new DocumentResponse::DeletedSelectedManipulatorPoints match case. When this DocumentResponse is emitted, it clears the Properties panel.

* Added Display trait implementation for DocumentResponse::DeletedSelectedManipulatorPoints. Updated imports in document_message_handler.rs to get the correct types for messages emitted from DocumentResponse::DeletedSelectedManipulatorPoints match case in process_message().

* Removed useless import. Capitalized comments for style consistency.

* Updated messages emitted to clear Properties panel by emitting LayoutMessage::SendLayout's instead, which update the backend widget state

* Revert inclusion of unused imports

---------

Co-authored-by: Keavon Chambers <keavon@keavon.com>

document-legacy/src/response.rs

@@ -21,6 +21,7 @@ pub enum DocumentResponse {
 	LayerChanged {
 		path: Vec<LayerId>,
 	},
+	DeletedSelectedManipulatorPoints,
 }
 
 impl fmt::Display for DocumentResponse {
@@ -31,6 +32,7 @@ impl fmt::Display for DocumentResponse {
 			DocumentResponse::CreatedLayer { .. } => write!(f, "CreatedLayer"),
 			DocumentResponse::LayerChanged { .. } => write!(f, "LayerChanged"),
 			DocumentResponse::DeletedLayer { .. } => write!(f, "DeleteLayer"),
+			DocumentResponse::DeletedSelectedManipulatorPoints { .. } => write!(f, "DeletedSelectedManipulatorPoints"),
 		}
 	}
 }