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/services/WhenClause.ts

@@ -0,0 +1,42 @@
+import { ContextKeyExpr, ContextKeyExpression } from './contextkey/contextkey';
+
+export default class WhenClause {
+
+	private expression_:string;
+	private validate_:boolean;
+	private rules_:ContextKeyExpression = null;
+
+	constructor(expression:string, validate:boolean) {
+		this.expression_ = expression;
+		this.validate_ = validate;
+	}
+
+	private createContext(ctx: any) {
+		return {
+			getValue: (key: string) => {
+				return ctx[key];
+			},
+		};
+	}
+
+	private get rules():ContextKeyExpression {
+		if (!this.rules_) {
+			this.rules_ = ContextKeyExpr.deserialize(this.expression_);
+		}
+
+		return this.rules_;
+	}
+
+	public evaluate(context:any):boolean {
+		if (this.validate_) this.validate(context);
+		return this.rules.evaluate(this.createContext(context));
+	}
+
+	public validate(context:any) {
+		const keys = this.rules.keys();
+		for (const key of keys) {
+			if (!(key in context)) throw new Error(`No such key: ${key}`);
+		}
+	}
+
+}