Desktop: Simplified and improve command service, and added command palette

- Commands "enabled" state is now expressed using a "when-clause" like in VSCode
- A command palette has been added to the Tools menu

ReactNativeClient/lib/commands/historyBackward.ts

@@ -4,27 +4,16 @@ import { _ } from 'lib/locale';
 export const declaration:CommandDeclaration = {
 	name: 'historyBackward',
 	label: () => _('Back'),
-	// iconName: 'fa-arrow-left',
 	iconName: 'icon-back',
 };
 
-interface Props {
-	hasBackwardNotes: boolean,
-}
-
 export const runtime = ():CommandRuntime => {
 	return {
-		execute: async (props:Props) => {
-			if (!props.hasBackwardNotes) return;
+		execute: async () => {
 			utils.store.dispatch({
 				type: 'HISTORY_BACKWARD',
 			});
 		},
-		isEnabled: (props:Props) => {
-			return props.hasBackwardNotes;
-		},
-		mapStateToProps: (state:any) => {
-			return { hasBackwardNotes: state.backwardHistoryNotes.length > 0 };
-		},
+		enabledCondition: 'historyhasBackwardNotes',
 	};
 };