Implement fill tool (#254)

* Implement fill tool

* Add fill tool shortcut

* Add getters and setters to styles

* Make fill tool act on the topmost layer clicked

* Refactor fill operation

* Refactor and unify selection tolerance

* Add mark_as_dirty function

* Fix getter names

core/document/src/operation.rs

@@ -1,4 +1,5 @@
 use crate::{
+	color::Color,
 	layers::{style, Layer},
 	LayerId,
 };
@@ -71,4 +72,8 @@ pub enum Operation {
 	ToggleVisibility {
 		path: Vec<LayerId>,
 	},
+	FillLayer {
+		path: Vec<LayerId>,
+		color: Color,
+	},
 }