React Tag Selector polish, i18n and tests

- Added icon-button UI code for the menubutton
- Upgrade to React 16 to allow non-standard attrs, such as `tooltiptext`
to support XUL tooltips
- Add i18n support for React UI elements
- Update tests for reactified tag selector

scripts/sass.js

@@ -28,7 +28,8 @@ async function getSass(source, options, signatures={}) {
 			if (compareSignatures(newFileSignature, signatures[f])) {
 				try {
 					await fs.access(dest, fs.constants.F_OK);
-					continue;
+					// TODO: Doesn't recompile on partial scss file changes, so temporarily disabled
+					// continue;
 				} catch (_) {
 					// file does not exists in build, fallback to browserifing
 				}