Restructure project directories (#333)

`/client/web` -> `/frontend`
`/client/cli` -> *delete for now*
`/client/native` -> *delete for now*
`/core/editor` -> `/editor`
`/core/document` -> `/graphene`
`/core/renderer` -> `/charcoal`
`/core/proc-macro` -> `/proc-macros` *(now plural)*

editor/src/frontend/frontend_message_handler.rs

@@ -0,0 +1,54 @@
+use crate::frontend::layer_panel::LayerPanelEntry;
+use crate::message_prelude::*;
+use crate::Color;
+use serde::{Deserialize, Serialize};
+
+pub type Callback = Box<dyn Fn(FrontendMessage)>;
+
+#[impl_message(Message, Frontend)]
+#[derive(PartialEq, Clone, Deserialize, Serialize, Debug)]
+pub enum FrontendMessage {
+	CollapseFolder { path: Vec<LayerId> },
+	ExpandFolder { path: Vec<LayerId>, children: Vec<LayerPanelEntry> },
+	SetActiveTool { tool_name: String },
+	SetActiveDocument { document_index: usize },
+	UpdateOpenDocumentsList { open_documents: Vec<String> },
+	DisplayConfirmationToCloseDocument { document_index: usize },
+	DisplayConfirmationToCloseAllDocuments,
+	UpdateCanvas { document: String },
+	UpdateLayer { path: Vec<LayerId>, data: LayerPanelEntry },
+	ExportDocument { document: String },
+	EnableTextInput,
+	DisableTextInput,
+	UpdateWorkingColors { primary: Color, secondary: Color },
+	SetCanvasZoom { new_zoom: f64 },
+	SetCanvasRotation { new_radians: f64 },
+}
+
+pub struct FrontendMessageHandler {
+	callback: crate::Callback,
+}
+
+impl FrontendMessageHandler {
+	pub fn new(callback: Callback) -> Self {
+		Self { callback }
+	}
+}
+
+impl MessageHandler<FrontendMessage, ()> for FrontendMessageHandler {
+	fn process_action(&mut self, message: FrontendMessage, _data: (), _responses: &mut VecDeque<Message>) {
+		(self.callback)(message)
+	}
+	advertise_actions!(
+		FrontendMessageDiscriminant;
+
+		CollapseFolder,
+		ExpandFolder,
+		SetActiveTool,
+		UpdateCanvas,
+		EnableTextInput,
+		DisableTextInput,
+		SetCanvasZoom,
+		SetCanvasRotation,
+	);
+}