Implement input hints based on the active tool state (#388)

* Hook up user input hints to display in the frontend status bar

Closes #171

* MVP hint system based on tool FSM

* Fix hints for Fill and Eyedropper tools

* Add icons for keyboard shortcuts

* Fix hints for Pen Tool

* Cleanup

editor/src/frontend/frontend_message_handler.rs

@@ -1,5 +1,6 @@
 use crate::document::layer_panel::{LayerPanelEntry, RawBuffer};
 use crate::message_prelude::*;
+use crate::misc::HintData;
 use crate::tool::tool_options::ToolOptions;
 use crate::Color;
 use serde::{Deserialize, Serialize};
@@ -11,6 +12,7 @@ pub enum FrontendMessage {
 	SetActiveTool { tool_name: String, tool_options: Option<ToolOptions> },
 	SetActiveDocument { document_index: usize },
 	UpdateOpenDocumentsList { open_documents: Vec<(String, bool)> },
+	UpdateInputHints { hint_data: HintData },
 	DisplayError { title: String, description: String },
 	DisplayPanic { panic_info: String, title: String, description: String },
 	DisplayConfirmationToCloseDocument { document_index: usize },