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.

tests/unit/application/Context/State/CategoryCollectionState.spec.ts

@@ -55,7 +55,7 @@ describe('CategoryCollectionState', () => {
             const collection = new CategoryCollectionStub();
             const sut = new CategoryCollectionState(collection);
             // act
-            const actual = sut.selection.totalSelected;
+            const actual = sut.selection.selectedScripts.length;
             // assert
             expect(actual).to.equal(0);
         });
@@ -68,7 +68,7 @@ describe('CategoryCollectionState', () => {
             // act
             sut.selection.selectAll();
             // assert
-            expect(sut.selection.totalSelected).to.equal(1);
+            expect(sut.selection.selectedScripts.length).to.equal(1);
             expect(sut.selection.isSelected(expectedScript.id)).to.equal(true);
         });
     });