SectorView key bindings

Re-implementation of #2212.

This has only had minimal testing. (In particular I might have accidentally reversed the direction of the sector view rotation controls.)

This also includes a small change of behaviour of the `KeyBinding::Matches(const SDL_Keysym*)` method, to match the behaviour of the `KeyBinding::IsActive()` method. I haven't tried to determine for sure whether the change in behaviour could cause other problems. When the key binding system is integrated with new-ui properly, we will have to think more carefully about what the correct behaviour is for handling bindings with modifiers.