fix script revert activating recommendation level

Reverting any single of the scripts from standard recommendation pool
shows "Standard" selection as selected which is wrong. This commit fixes
it, refactors selection handling in a separate class and it also adds
missing tests. It removes UserSelection.totalSelected propertty in favor of using
UserSelection.selectedScripts.length to provide unified way of accessing
the information.

src/application/Context/State/Selection/IUserSelection.ts

@@ -6,7 +6,6 @@ import { IEventSource } from '@/infrastructure/Events/IEventSource';
 export interface IUserSelection {
     readonly changed: IEventSource<ReadonlyArray<SelectedScript>>;
     readonly selectedScripts: ReadonlyArray<SelectedScript>;
-    readonly totalSelected: number;
     areAllSelected(category: ICategory): boolean;
     isAnySelected(category: ICategory): boolean;
     removeAllInCategory(categoryId: number): void;

src/application/Context/State/Selection/UserSelection.ts

@@ -101,10 +101,6 @@ export class UserSelection implements IUserSelection {
         return this.scripts.getItems();
     }
 
-    public get totalSelected(): number {
-        return this.scripts.getItems().length;
-    }
-
     public selectAll(): void {
         for (const script of this.collection.getAllScripts()) {
             if (!this.scripts.exists(script.id)) {