misc: fix some frameless window hit-test issues

- Moves the plugin settings close(on GNU/Linux and Windows) to the left to avoid a visual conflict with the system buttons.
- Adds hit testability to BackButton.qml

Note: this commit does not address the style difference in the back buttons thoughout the onboarding pages.
Change-Id: Ia4a2ce2a1228888e3c3101f3472fbe1843b4a522