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/misc/mod.rs

@@ -2,7 +2,9 @@
 pub mod macros;
 pub mod derivable_custom_traits;
 mod error;
+pub mod hints;
 pub mod test_utils;
 
 pub use error::EditorError;
+pub use hints::*;
 pub use macros::*;