(refactor) ApplicationService: Use Application instead of ApplicationFixture.

[hastebrot]

• Use Application (instead of ApplicationFixture) in ApplicationService and FxToolkit.
• Move ApplicationFixture and ApplicationAdapter from testfx-core to testfx-junit.

This changes ApplicationAdapter to extend Application instead of implementing ApplicationFixture. We need a ApplicationFixture interface in testfx-junit to let ApplicationRule extend FxRobot (single inheritance of Java).

// before.
class ApplicationAdapter implements ApplicationFixture {}

// after.
class ApplicationAdapter extends Application {}

cc: @johnzeringue (this should not affect the behaviour of ApplicationRule).

[hastebrot]

This also allows us to implement a complete ApplicationServiceImplTest.