Accessible modals (#560)

Improve the accessibility of our modals (the color picker and the export dialog)

Implement a focus trap so that tapping through the controls inside them don't escape to outer elements, it also allows to close the modals with the "Escape" key.

src/keys.ts

@@ -12,6 +12,7 @@ export const KEYS = {
       ? "metaKey"
       : "ctrlKey";
   },
+  TAB: "Tab",
 };
 
 export function isArrowKey(keyCode: string) {