Consistent set nav target click

Makes clicking/un-clicking ships, planets, orbital stations behave the same.
No holding down SHIFT required for bodies anymore.

Fixes not being able to un-click planets.

With this PR the behaviour will be same as in SystemInfoView when clicking star or orbital station.

I also renamed `SystemView::OnClickShipLabel` to `SystemView::OnClickShip` to highlight the similarity with `SystemView::OnClickBody`. There's some code duplication between them, that maybe could be eliminated.