Restructure the entire editor codebase to consistently match the message hierarchy

Closes #744

editor/src/messages/tool/tool_messages/mod.rs

@@ -0,0 +1,15 @@
+pub mod artboard_tool;
+pub mod ellipse_tool;
+pub mod eyedropper_tool;
+pub mod fill_tool;
+pub mod freehand_tool;
+pub mod gradient_tool;
+pub mod line_tool;
+pub mod navigate_tool;
+pub mod path_tool;
+pub mod pen_tool;
+pub mod rectangle_tool;
+pub mod select_tool;
+pub mod shape_tool;
+pub mod spline_tool;
+pub mod text_tool;